Update to latest libgit2.
Update to latest merge option changes.
Update to latest `oid` -> `id` changes.
Update to latest `git_buf` API changes.

This also fixes two memory leaks in `rb_git_prettify_message` and
`rb_git_repo_discover`.
Update merge documentation. [ci:skip]
Valid C99.
Merge pull request #306 from libgit2/arthur/update-to-latest-libgit2

Update to the latest libgit2 changes.