dirstate: fold paths using the just added util.realpath() function.

Using the one true canonical path of a directory entry allows
equivalent paths to be treated gracefully. Equivalent means, in this
case, differing directory entries resulting in the same, unique file
system link to a file.

tests/test-path-normalization

@@ -0,0 +1,4 b''
+#!/bin/sh
+
+hg clone --quiet $TESTDIR/test-path-normalization.hg t
+exec hg st -R t

mercurial/dirstate.py

@@ -59,7 +59,7 b' class dirstate(object):'
     def _foldmap(self):
         f = {}
         for name in self._map:
-            f[os.path.normcase(name)] = name
+            f[util.realpath(self._join(name))] = name
         return f
 
     @propertycache
@@ -340,7 +340,7 b' class dirstate(object):'
             self._ui.warn(_("not in dirstate: %s\n") % f)
 
     def _normalize(self, path, knownpath):
-        norm_path = os.path.normcase(path)
+        norm_path = util.realpath(self._join(path))
         fold_path = self._foldmap.get(norm_path, None)
         if fold_path is None:
             if knownpath or not os.path.exists(os.path.join(self._root, path)):