dirstate: use list comprehension to get a list of keys

We have used dict.keys() which returns a dict_keys() object instead
of list on Python 3. So this patch replaces that with list comprehension
which works both on Python 2 and 3.

mercurial/dirstate.py

@@ -1079,7 +1079,7 b' class dirstate(object):'
             # a) not matching matchfn b) ignored, c) missing, or d) under a
             # symlink directory.
             if not results and matchalways:
-                visit = dmap.keys()
+                visit = [f for f in dmap]
             else:
                 visit = [f for f in dmap if f not in results and matchfn(f)]
             visit.sort()