Update libgit2.
++libgit2
Always force reloading of the index from -refresh:
Strip out refspec removal functionality to match libgit2

See libgit2/libgit2@266af6d81960144334c16e061f1d30f94f8a1b46 for the
rationale.
Verify that reference.name is not nil
Merge pull request #291 from libgit2/update-libgit2

Update libgit2.