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

Hi,

libgit2 v0.21.0 was just released, and there have been some changes to the API.

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

* 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.
]]]