Update to upstream libgit2
++libgit2 to latest development
Update GTObjectDatabase for libgit2 changes
++libgit2 to is-empty-fix to fix unit test failures
++libgit2
Merge pull request #244 from libgit2/update-libgit2

Update libgit2 to latest development
Add an OID property to GTDiffFile.
Minor cleanup
Merge pull request #249 from libgit2/gtdifffile-oid

Add an OID property to GTDiffFile