dirstate: test normalize is truthy instead of using a no-op lambda

hg perfstatus -u on a working directory with 170,000 files, without this
change:
! wall 1.869404 comb 1.850000 user 1.170000 sys 0.680000 (best of 6)

With this change:
! wall 1.839561 comb 1.830000 user 1.120000 sys 0.710000 (best of 6)

mercurial/dirstate.py

@@ -605,7 +605,7 b' class dirstate(object):'
             normalize = self._normalize
             skipstep3 = False
         else:
-            normalize = lambda x, y, z: x
+            normalize = None
 
         files = sorted(match.files())
         subrepos.sort()
@@ -626,7 +626,10 b' class dirstate(object):'
 
         # step 1: find all explicit files
         for ff in files:
-            nf = normalize(normpath(ff), False, True)
+            if normalize:
+                nf = normalize(normpath(ff), False, True)
+            else:
+                nf = normpath(ff)
             if nf in results:
                 continue
 
@@ -676,7 +679,10 b' class dirstate(object):'
                     continue
                 raise
             for f, kind, st in entries:
-                nf = normalize(nd and (nd + "/" + f) or f, True, True)
+                if normalize:
+                    nf = normalize(nd and (nd + "/" + f) or f, True, True)
+                else:
+                    nf = nd and (nd + "/" + f) or f
                 if nf not in results:
                     if kind == dirkind:
                         if not ignore(nf):