Introduce Repository.CherryPick.

Okay so, this is pretty weird, but from the looks of it, CherryPick has lots of copied stuff from RevertOptions and GitRevertOpts (basically a 1:1 mirror.)

Also, the unit test are not yet complete. Imo, they should include almost the same tests as RevertFixture. But using MergeRepo as the test repo since it is already branched.
Why should we allow null? `Merge()` doesn't, does it?