This branch implements Git repository access for Subversion.
The implementation is a new RA module called libsvn_ra_git.

$Id$

= Design =

Git repository are accessed via the following URL schemes:

  git://          native git protocol
  git+file://     local git repository
  git+http://     git protocol via http
  git+https://    git protocol via https

All git operations are performed with libgit2. This code works with
libgit2 v0.27.x. See http://libgit2.github.com/

A bare git repository is stored in .svn/git. The svn working copy
acts like the git working tree. For commands which don't use a 
working copy (e.g. svn info URL) the git repository is stored
at a temporary location.

svn follows the linear history of the git branch it mirrors.
That means svn only sees the first parent of every git commit.

All commits along this linear history are mapped to Subversion
revision numbers. When the svn client asks for a path at a specific
revision, the git RA layer returns the object at the same path in
the corresponding git commit.

Commits are made to the local git repository with a subsequent
attempt to push to the git repository's remote (i.e. the repository
the initial clone was made from). If this push fails, a hard reset
is performed to undo the local commit, and new objects are fetched
from the remote. The svn client must then run an update to merge the
newly fetched objects and try again.

= Status =

The following subcommands work with git repositories:

   blame (praise, annotate, ann)
   cat
   checkout (co)
   export
   diff (di)
   info
   list (ls)
   log
   status (stat, st)
   switch (sw)
   update (up)
   relocate
   youngest

One limitation of checkout/export is that they currently expect
to get a branch called 'master'. This should be made configurable.
Unfortunately, there is no standard representation of branch names
in git URLs. Some people use # as a separator in a URL:
  git://git.example.com/myrepository#master
We could either implement this scheme, or add a --branch option
to checkout/export for this purpose.

The following still need to be implemented:

   commit (ci)
   copy (cp)
   delete (del, remove, rm)
   import
   mkdir
   move (mv, rename, ren)

The following still need to be implemented (but I'm not yet sure how);

   propdel (pdel, pd)
   propedit (pedit, pe)
   propget (pget, pg)
   proplist (plist, pl)
   propset (pset, ps)

The following will not be implemented:

   lock
   merge
   mergeinfo
   unlock

Lock/unlock are not needed in a distributed system like git.

Implementing merge support is just a lot of work for no gain.
The native merge capabilities of git far exceed those of svn anyway.
Users can simply use a native git client to merge branches.
Subversion's mergeinfo cannot accurately represent the commit DAG
of a git repository.

The following are working-copy only and thus don't use the RA layer:

   patch
   resolve
   resolved
   revert
   upgrade

= Other TODO items =

- Progress output for git operations (currently only visible in debug builds)
- Map libgit2 error codes to svn error codes
- Make libgit2 support optional at build time (it's LGPL licensed)
- Support git+ssh:// (requires libssh support in ligit2)
- Test suite integration (so "make check" can be run with ra_git)
- Documentation updates
- Look for git subcommands which would be worth adding to 'svn'