dirstate.walk: fast path none-seen + match-always case for step 3

This case is a common one -- e.g. `hg diff`.

For a repository with 170,000 files, this speeds up perfstatus from 0.95
seconds to 0.88.

mercurial/dirstate.py

@@ -705,7 +705,12 b' class dirstate(object):'
 
         # step 3: report unseen items in the dmap hash
         if not skipstep3 and not exact:
-            visit = sorted([f for f in dmap if f not in results and matchfn(f)])
+            if not results and matchalways:
+                visit = dmap.keys()
+            else:
+                visit = [f for f in dmap if f not in results and matchfn(f)]
+            visit.sort()
+
             if unknown:
                 # unknown == True means we walked the full directory tree above.
                 # So if a file is not seen it was either a) not matching matchfn