Bump libgit2 to ce8822cb409917d0201f359cabe3ae55d25895da
`git_remote_supported_url()` does not exist anymore.
`git_merge_head` is now `git_annotated_commit`.
`git_remote_load` is now `git_remote_lookup`.
`git_threads_*` is now `git_libgit2_*`.
Replace `Remote#rename!` with `RemoteCollection#rename`.
Merge pull request #435 from libgit2/arthur/update-libgit2

Update to latest libgit2