mq: prefer a loop to a double-for list comprehension

The [x for y in l for x in y] syntax is nigh-incomprehensible, and this
is a particularly easy case to expand into a loop since there's no 'if'
condition in the list comprehension.

hgext/mq.py

@@ -1204,7 +1204,9 b' class queue(object):'
         diffopts = self.diffopts()
         wlock = repo.wlock()
         try:
-            heads = [h for hs in repo.branchmap().itervalues() for h in hs]
+            heads = []
+            for hs in repo.branchmap().itervalues():
+                heads.extend(hs)
             if not heads:
                 heads = [nullid]
             if repo.dirstate.p1() not in heads and not exact: