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.
#require test-repo pyflakes
Copied from Mercurial core (60ee2593a270)
$ cd "`dirname "$TESTDIR"`"
run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)
$ hg locate -I 'set:(**.py or grep("^#!.*python")) - removed()' 2>/dev/null \
> | xargs pyflakes 2>/dev/null