Update to libgit2@0.21.1.
Update to libgit2@0.21.1.

Acidburn0zzz/rugged,saraid/rugged,libgit2/rugged,libgit2/rugged,saraid/rugged,libgit2/rugged,Acidburn0zzz/rugged,Acidburn0zzz/rugged,tpickett66/rugged,tpickett66/rugged,tpickett66/rugged,saraid/rugged