oidarray: introduce `git_oidarray_dispose`
Since users are disposing the _contents_ of the oidarray, not freeing
the oidarray itself, the proper cleanup function is
`git_oidarray_dispose`. Deprecate `git_oidarray_free`.
merge: expose multiple merge bases
We always calculate multiple merge bases, but up to now we had only
exposed the "best" merge base.
Introduce git_oidarray which analogously to git_strarray lets us return
multiple ids.