mq: drop the use of mq.queue.qparent in mq.queue.strip

Same as in the previous changeset, rev is never `None`. We can just copy the two
relevant lines in in `queue.strip`. This help having `queue.strip` independent
from `queue`. One further step toward the extraction of `strip` in an independent
extension. (As discussed in issue3824).

hgext/mq.py

@@ -1111,7 +1111,9 b' class queue(object):'
 
             if update:
                 checklocalchanges(repo, force=force)
-                urev = self.qparents(repo, revs[0])
+                urev, p2 = repo.changelog.parents(revs[0])
+                if p2 != nullid and p2 in [x.node for x in self.applied]:
+                    urev = p2
                 hg.clean(repo, urev)
                 repo.dirstate.write()