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).

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t