dirstate._normalize: don't construct dirfoldmap if not necessary

Constructing the dirfoldmap is expensive, so if there's a hit in the
filefoldmap, don't construct the directory foldmap.

This helps with cases like 'hg add foo' where foo is already tracked: for a
large repository, the operation goes from 1.5 seconds to 1.2 (which is still
way too much, but that's a matter for another day.)

mercurial/dirstate.py

@@ -509,8 +509,9 b' class dirstate(object):'
 
     def _normalize(self, path, isknown, ignoremissing=False, exists=None):
         normed = util.normcase(path)
-        folded = self._filefoldmap.get(normed,
-                                       self._dirfoldmap.get(normed, None))
+        folded = self._filefoldmap.get(normed, None)
+        if folded is None:
+            folded = self._dirfoldmap.get(normed, None)
         if folded is None:
             if isknown:
                 folded = path