Hmm... if we can make qpush/qpop safe to use even with local changes,
then the --force flag would not be needed :-)

It would be nice if MQ would look ahead and see if the patch conflicts
with local changes (not just on a file basis, but on a hunk-basis).
But I don't think it's mandatory since it is a --force option, and
users should not use --force if they don't know what they are doing.