dirstate: cache util.normcase while constructing the foldmap

This is a small win on OS X. hg perfdirstatefoldmap:

before: wall 0.399708 comb 0.410000 user 0.390000 sys 0.020000 (best of 25)
after:  wall 0.386331 comb 0.390000 user 0.370000 sys 0.020000 (best of 25)

mercurial/dirstate.py

@@ -84,11 +84,12 b' class dirstate(object):'
     @propertycache
     def _foldmap(self):
         f = {}
+        normcase = util.normcase
         for name, s in self._map.iteritems():
             if s[0] != 'r':
-                f[util.normcase(name)] = name
+                f[normcase(name)] = name
         for name in self._dirs:
-            f[util.normcase(name)] = name
+            f[normcase(name)] = name
         f['.'] = '.' # prevents useless util.fspath() invocation
         return f