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