Update to latest libgit2.
Update to latest libgit2 reference changes.
Reference.create now takes a custom reflog message and/or signature.
Reference#set_target now takes a custom reflog message and/or signature.
Drop `Reference#log!`.

Pass custom log messages and/or signatures to `Reference.create` or
`Reference#set_target` instead.
Merge pull request #305 from libgit2/arthur/reflog-changes

Update to latest libgit2 reflog changes