dirstate: skip step 3 in walk if nothing new will match

nothing will ever match on match.never
nothing new will match on match.exact (all found in step 1)
nothing new will match on match.match when
  there is no pattern and
  there is no direcory in pats

mercurial/dirstate.py

@@ -461,6 +461,23 b' class dirstate(object):'
         work = []
         wadd = work.append
 
+        if match.anypats():
+            #match.match with patterns
+            dostep3 = True
+            nomatches = False
+        elif not match.files():
+            #match.always or match.never
+            dostep3 = matchfn('')
+            nomatches = not dostep3
+        else:
+            #match.exact or match.match without pattern
+            dostep3 = False
+            nomatches = matchfn == match.exact
+
+        if nomatches:
+            #skip step 2
+            dirignore = util.always
+
         files = set(match.files())
         if not files or '.' in files:
             files = ['']
@@ -476,6 +493,7 b' class dirstate(object):'
                 st = lstat(join(nf))
                 kind = getkind(st.st_mode)
                 if kind == dirkind:
+                    dostep3 = True
                     if nf in dmap:
                         #file deleted on disc but still in dirstate
                         results[nf] = None
@@ -497,6 +515,7 b' class dirstate(object):'
                         keep = True
                         break
                     elif fn.startswith(prefix):
+                        dostep3 = True
                         keep = True
                         break
                 if not keep:
@@ -541,11 +560,12 b' class dirstate(object):'
                         results[nf] = None
 
         # step 3: report unseen items in the dmap hash
-        visit = sorted([f for f in dmap if f not in results and matchfn(f)])
-        for nf, st in zip(visit, util.statfiles([join(i) for i in visit])):
-            if not st is None and not getkind(st.st_mode) in (regkind, lnkkind):
-                st = None
-            results[nf] = st
+        if dostep3 and not nomatches:
+            visit = sorted([f for f in dmap if f not in results and matchfn(f)])
+            for nf, st in zip(visit, util.statfiles([join(i) for i in visit])):
+                if not st is None and not getkind(st.st_mode) in (regkind, lnkkind):
+                    st = None
+                results[nf] = st
 
         del results['.hg']
         return results