dirstate: introduce function to normalize just filenames

This will be used in upcoming patches to stop generating the set of directories
in many common cases.

mercurial/dirstate.py

@@ -490,6 +490,17 b' class dirstate(object):'
 
         return folded
 
+    def _normalizefile(self, path, isknown, ignoremissing=False, exists=None):
+        normed = util.normcase(path)
+        folded = self._foldmap.get(normed, None)
+        if folded is None:
+            if isknown:
+                folded = path
+            else:
+                folded = self._discoverpath(path, normed, ignoremissing, exists,
+                                            self._foldmap)
+        return folded
+
     def _normalize(self, path, isknown, ignoremissing=False, exists=None):
         normed = util.normcase(path)
         folded = self._foldmap.get(normed, None)