CommitService: implement ListLastCommitForTrees
Despite its name, this method does not have much to do with Git trees in themselves: for each given path, it provides the commit that lastly changed it.
Accidentally, we already have code on the Rails side working around the lack of this one (not landed at the time of this writing), just by using the Git conversion that still is happening. Therefore we don't really need this method to claim the HGitaly1 milestone.