metaedit: allow operations on merge commits with some conditions As with fold (see the previous patch), it's allowed to metaedit a merge commit or a set of commits including merge commits (with --fold) as long as there are less than 2 parents of the set not included in the said set.

fold: allow operations on merge commits with some conditions It's possible to fold revision chains that include a single merge commit: just fold everything into the merge commit while saving its other parent (so it continues being a merge commit). It's also possible to fold revisions that include multiple merge commits, on the condition that they merge with not more than 2 external changesets (i.e. a changesets that aren't going to be folded).