[PATCH] On the 'ra-git' branch: update to libgit2 v0.21

Hi,

Based on feedback from Daniel Shahaf, I've made the build system care
about which version of libgit2 we're trying to link against. I copied
what apache.m4 was doing, which I figured was a good template.

[[[
On the ‘ra-git’ branch: update to libgit2 v0.21

* build/ac-macros/libgit2.m4:
  Check for the exact version we need to build against

* subversion/libsvn_ra_git/ra_plugin.c:
  (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.
  (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.
]]]