[MERGE] Add revert --forget-merges

[Martin Pool]

> Just 3 days ago I looking for such mechanism,
> when I split my big patch on smaller chunks.
> (You probably read my mind :-D).
> Thank you!

Thanks!  I also meant to ask, and I'll ask you now: do you think the
name and help text is sufficiently clear?  I was going to add it to
the user guide but couldn't find a good place; I probably need to add
a new section about revert and how it interacts with merge.

-- 
Martin