reorg all the pages about rcs backends. Fix all links

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+A few bits about the RCS backends
+
+[[toc ]]
+
+## Terminology
+
+``web-edit'' means that a page is edited by using the web (CGI) interface
+as opposed to using a editor and the RCS interface.
+
+
+## [[svn]]
+
+Subversion was the first RCS to be supported by ikiwiki.
+
+### How does it work internally?
+
+Master repository M.
+
+RCS commits from the outside are installed into M.
+
+There is a working copy of M (a checkout of M): W.
+
+HTML is generated from W.  rcs_update() will update from M to W.
+
+CGI operates on W.  rcs_commit() will commit from W to M.
+
+For all the gory details of how ikiwiki handles this behind the scenes,
+see [[commit-internals]].
+
+You browse and web-edit the wiki on W.
+
+W "belongs" to ikiwiki and should not be edited directly.
+
+
+## [darcs](http://darcs.net/) (not yet included)
+
+Support for using darcs as a backend is being worked on by [Thomas
+Schwinge](mailto:tschwinge@gnu.org), although development is on hold curretly.
+There is a patch in [[todo/darcs]].
+
+### How will it work internally?
+
+``Master'' repository R1.
+
+RCS commits from the outside are installed into R1.
+
+HTML is generated from R1.  HTML is automatically generated (by using a
+``post-hook'') each time a new change is installed into R1.  It follows
+that rcs_update() is not needed.
+
+There is a working copy of R1: R2.
+
+CGI operates on R2.  rcs_commit() will push from R2 to R1.
+
+You browse the wiki on R1 and web-edit it on R2.  This means for example
+that R2 needs to be updated from R1 if you are going to web-edit a page,
+as the user otherwise might be irritated otherwise...
+
+How do changes get from R1 to R2?  Currently only internally in
+rcs\_commit().  Is rcs\_prepedit() suitable?
+
+It follows that the HTML rendering and the CGI handling can be completely
+separated parts in ikiwiki.
+
+What repository should [[RecentChanges]] and [[History]] work on?  R1?
+
+#### Rationale for doing it differently than in the Subversion case
+
+darcs is a distributed RCS, which means that every checkout of a
+repository is equal to the repository it was checked-out from.  There is
+no forced hierarchy.
+
+R1 is nevertheless called the master repository.  It's used for
+collecting all the changes and publishing them: on the one hand via the
+rendered HTML and on the other via the standard darcs RCS interface.
+
+R2, the repository the CGI operates on, is just a checkout of R1 and
+doesn't really differ from the other checkouts that people will branch
+off from R1.
+
+(To be continued.)
+
+#### Another possible approach
+
+Here's what I (tuomov) think, would be a “cleaner” approach:
+
+ 1. Upon starting to edit, Ikiwiki gets a copy of the page, and `darcs changes --context`.
+     This context _and_ the present version of the page are stored in as the “version” of the
+     page in a hidden control of the HTML.
+     Thus the HTML includes all that is needed to generate a patch wrt. to the state of the
+     repository at the time the edit was started. This is of course all that darcs needs.
+ 2. Once the user is done with editing, _Ikiwiki generates a patch bundle_ for darcs.
+     This should be easy with existing `Text::Diff` or somesuch modules, as the Web edits
+     only concern single files. The reason why the old version of the page is stored in
+     the HTML (possibly compressed) is that the diff can be generated.
+ 3. Now this patch bundle is applied with `darcs apply`, or sent by email for moderation…
+     there are many possibilities.
+
+This approach avoids some of the problems of concurrent edits that the previous one may have,
+although there may be conflicts, which may or may not propagate to the displayed web page.
+(Unfortunately there is not an option to `darcs apply` to generate some sort of ‘confliction resolution
+bundle’.) Also, only one repository is needed, as it is never directly modified
+by Ikiwiki. 
+
+This approach might be applicable to other distributed VCSs as well, although they're not as oriented
+towards transmitting changes with standalone patch bundles (often by email) as darcs is.
+
+> The mercurial plugin seems to just use one repo and edit it directly - is
+> there some reason that's okay there but not for darcs? I agree with tuomov
+> that having just the one repo would be preferable; the point of a dvcs is
+> that there's no difference between one repo and another. I've got a
+> darcs.pm based on mercurial.pm, that's almost usable... --bma
+
+>> IMHO it comes down to whatever works well for a given RCS. Seems like
+>> the darcs approach _could_ be done with most any distributed system, but
+>> it might be overkill for some (or all?) While there is the incomplete darcs
+>> plugin in [[todo/darcs]], if you submit one that's complete, I will
+>> probably accept it into ikiwiki.. --[[Joey]]
+
+## [[Git]]
+
+Regarding the Git support, Recai says:
+
+I have been testing it for the past few days and it seems satisfactory.  I
+haven't observed any race condition regarding the concurrent blog commits
+and it handles merge conflicts gracefully as far as I can see.
+
+As you may notice from the patch size, GIT support is not so trivial to
+implement (for me, at least).  Being a fairly fresh code base it has some
+bugs.  It also has some drawbacks (especially wrt merge which was the hard
+part).  GIT doesn't have a similar functionality like 'svn merge -rOLD:NEW
+FILE' (please see the relevant comment in mergepast for more details), so I
+had to invent an ugly hack just for the purpose.
+
+By design, Git backend uses a "master-clone" repository pair approach in contrast
+to the single repository approach (here, _clone_ may be considered as the working
+copy of a fictious web user).  Even though a single repository implementation is
+possible, it somewhat increases the code complexity of backend (I couldn't figure
+out a uniform method which doesn't depend on the prefered repository model, yet).
+By exploiting the fact that the master repo and _web user_'s repo (`srcdir`) are all
+on the same local machine, I suggest to create the latter with the "`git clone -l -s`"
+command to save disk space.
+
+Note that, as a rule of thumb, you should always put the rcs wrapper (`post-update`)
+into the master repository (`.git/hooks/`) as can be noticed in the Git wrappers of
+the sample [[ikiwiki.setup]].
+
+## [[Mercurial]]
+
+The Mercurial backend is still in a early phase, so it may not be mature 
+enough, but it should be simple to understand and use.
+
+As Mercurial is a distributed RCS, it lacks the distinction between 
+repository and working copy (every wc is a repo).
+
+This means that the Mercurial backend uses directly the repository as 
+working copy (the master M and the working copy W described in the svn 
+example are the same thing).
+
+You only need to specify 'srcdir' (the repository M) and 'destdir' (where
+the HTML will be generated).
+
+Master repository M.
+
+RCS commit from the outside are installed into M.
+
+M is directly used as working copy (M is also W).
+
+HTML is generated from the working copy in M. rcs_update() will update 
+to the last committed revision in M (the same as 'hg update').
+If you use an 'update' hook you can generate automatically the HTML
+in the destination directory each time 'hg update' is called.
+
+CGI operates on M. rcs_commit() will commit directly in M.
+
+If you have any question or suggestion about the Mercurial backend 
+please refer to [Emanuele](http://nerd.ocracy.org/em/)
+
+## [[tla]]
+
+## rcs
+
+There is a patch that needs a bit of work linked to from [[todo/rcs]].
+
+## [[Monotone]]
+
+In normal use, monotone has a local database as well as a workspace/working copy.
+In ikiwiki terms, the local database takes the role of the master repository, and
+the srcdir is the workspace.  As all monotone workspaces point to a default
+database, there is no need to tell ikiwiki explicitly about the "master" database.  It
+will know.
+
+The backend currently supports normal committing and getting the history of the page.
+To understand the parallel commit approach, you need to understand monotone's
+approach to conflicts:
+
+Monotone allows multiple micro-branches in the database.  There is a command,
+`mtn merge`, that takes the heads of all these branches and merges them back together
+(turning the tree of branches into a dag).  Conflicts in monotone (at time of writing)
+need to be resolved interactively during this merge process.
+It is important to note that having multiple heads is not an error condition in a
+monotone database.  This condition will occur in normal use.  In this case
+'update' will choose a head if it can, or complain and tell the user to merge.
+
+For the ikiwiki plugin, the monotone ikiwiki plugin borrows some ideas from the svn ikiwiki plugin.
+On prepedit() we record the revision that this change is based on (I'll refer to this as the prepedit revision).  When the web user
+saves the page, we check if that is still the current revision.  If it is, then we commit.
+If it isn't then we check to see if there were any changes by anyone else to the file
+we're editing while we've been editing (a diff bewteen the prepedit revision and the current rev).
+If there were no changes to the file we're editing then we commit as normal.
+
+It is only if there have been parallel changes to the file we're trying to commit that
+things get hairy.  In this case the current approach is to
+commit the web changes as a branch from the prepedit revision.  This
+will leave the repository with multiple heads.  At this point, all data is saved.
+The system then tries to merge the heads with a merger that will fail if it cannot
+resolve the conflict.  If the merge succeeds then everything is ok.
+
+If that merge failed then there are conflicts.  In this case, the current code calls
+merge again with a merger that inserts conflict markers.  It commits this new
+revision with conflict markers to the repository.  It then returns the text to the
+user for cleanup.  This is less neat than it could be, in that a conflict marked
+revision gets committed to the repository.
diff --git a/doc/rcs/git.mdwn b/doc/rcs/git.mdwn
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+[[meta title="Git"]]
+
+[Git](http://git.or.cz) is a distributed revison control system originally developed for the Linux kernel. Ikiwiki supports storing a wiki in git.
+
+Ikiwiki can run as a post-update hook to update a wiki whenever commits
+come in. When running as a [[cgi]] with Git, ikiwiki automatically
+commits edited pages, and uses the Git history to generate the [[RecentChanges]] page.
\ No newline at end of file
diff --git a/doc/rcs/mercurial.mdwn b/doc/rcs/mercurial.mdwn
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+[Mercurial](http://selenic.com/mercurial) is a distributed revison control
+system developed by Matt Mackall. Ikiwiki supports storing a wiki in a
+mercurial repository.
+
+Ikiwiki can run as a post-update hook to update a wiki whenever commits
+come in. When running as a [[cgi]] with Mercurial, ikiwiki automatically
+commits edited pages, and uses the Mercurial history to generate the
+[[RecentChanges]] page.
diff --git a/doc/rcs/monotone.mdwn b/doc/rcs/monotone.mdwn
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+[monotone](http://monotone.ca/) is a distributed revision control system.
+Ikiwiki supports storing a wiki in tla.
+
+This requires the Monotone perl module from the monotone contrib/ directory
+to be installed. In particlar, it needs version 0.03 or higher of that module.
+It is available from the monotone source repository at:
+<http://viewmtn.angrygoats.net/branch/changes/net.venge.monotone>
diff --git a/doc/rcs/svn.mdwn b/doc/rcs/svn.mdwn
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+[Subversion](http://subversion.tigris.org/) is a revision control system. While ikiwiki is relatively
+independent of the underlying revision control system, and can easily be
+used without one, using it with Subversion is recommended since it's how
+the author uses it.
+
+Ikiwiki can run as a [[post-commit]] hook to update a wiki whenever commits
+come in. When running as a [[cgi]] with Subversion, ikiwiki automatically
+commits edited pages to the subversion repostory, and uses the Subversion
+log to generate the [[RecentChanges]] page.
diff --git a/doc/rcs/svn/discussion.mdwn b/doc/rcs/svn/discussion.mdwn
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+If the user interrupts the page loading during the running of `svn commit`,
+the repository will be left in an inconsistent state. The probability of
+this happening increases with the size of the repository and the number of
+plugins installed, because these both affect how long the post-commit hook
+takes to run. (The core issue, I guess, is that we're abusing the concept
+of a "working copy" by giving everybody the same one). Here are the main
+solutions that I can see: (1) CGI queues commits so that a single process
+can act upon them sequentially, or (2) optionally divorce the `ikiwiki
+--refresh` from the `svn commit` so that commits happen faster. -- [[Ben]]
+
+I'm not aware of web servers, at least apache, killing cgi processes when
+the user stops a page load. If this is happening ikiwiki should be able to
+avoid it by blocking whatever signal is causing it to terminate. --[[Joey]]
diff --git a/doc/rcs/tla.mdwn b/doc/rcs/tla.mdwn
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+[tla](http://wiki.gnuarch.org/) is an implementation of the
+[GNU](http://www.gnu.org/) [Arch](http://www.gnuarch.org/) revision control
+system. Ikiwiki supports storing a wiki in tla.
+
+Ikiwiki can run as a [[post-commit]] hook to update a wiki whenever commits
+come in. When running as a [[cgi]] with tla, ikiwiki automatically
+commits edited pages to the Arch repostory, and uses the Arch
+log to generate the [[RecentChanges]] page.
+
+Note that the tla support needs the [[cpan MailTools]] perl module.