This conversion follows the same pattern as CherryPick: loop through
the commits to merge and add one op to the batch per commit. One
complicating factor not present in CherryPick is delegating the rebase
work to RebaseChangeOp.
The reason this is only "mostly" converted is that for various reasons
(noted in TODOs), we can't yet use the Repository/RevWalk/Inserter
from the RepoContext. I expect this to be fixed in the next phase of
migrating MergeOp, where we hoist the BatchUpdate construction above
the SubmitStrategy level. For now, this is still safe because the
various methods in places like MergeUtil are still aggressively
flushing the inserter.
Change-Id: I8a26183dd60a2f2fd4e7478e43de258a781b3126
f2839d7c16