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.

In theory we could have this for octopus merges too, but that doesn't seem to be how our function wants to behave, as it's only ever taking one at a time.

/cc @arthurschreiber