[PATCH] On the 'ra-git' branch: update to libgit2 v0.22

Hi,

The git RA layer still expects to be built against v0.20 of libgit2, which has been
unsupported for quite a while now. This patch brings the code up to date to work
against the v0.22 API.

[[[
On the 'ra-git' branch: update to libgit2 v0.22

* BRANCH-README:
  Specify we build against libgit2 v0.22.

* autogen.sh:
  Fix the LIBTOOL_CONFIG replacement when overriding the config file path

* subversion/libsvn_ra_git/ra_plugin.c:
 (do_libgit_init): the init function was renamed to git_libgit2_init().
 (split_url): remotes which are not in the config are now called anonymous.
 (do_git_fetch): pass NULL to git_remote_fetch() to get default reflog entries.
 (svn_ra_git__open): git_remote_load() was renamed git_remote_lookup().
 (remote_progress_cb, svn_ra_git__open): the progress callback has been
   renamed to sideband_progress. Rename our callback function to
   remote_sideband_progress_cb to match libgit2 naming.
 (compare_git_tree_entries): git_tree_entry_byoid is now git_tree_entry_byid.

* subversion/libsvn_ra_git/reporter.c:
 (blob_relatedness_cb): delta file field oid has been renamed to id.
]]]

   cmn