icasefs: use case preserved root for 'util.fspath()' invocation (issue3302)

path to repo root may contains case sensitive part, even though repo
is located in case insensitive filesystem: e.g. repo in FAT32 device
mounted on Unix.

so, case normalized root causes failure of stat(2).

this patch uses case preserved root for 'util.fspath()' invocation to
avoid this problem.

case preserved root for 'util.fspath()' may decrease efficiency of
fspath cache, but 'util.fspath()' is currently called only from
dirstate, so this fix has less impact.

mercurial/dirstate.py

@@ -78,10 +78,6 b' class dirstate(object):'
         return self._copymap
 
     @propertycache
-    def _normroot(self):
-        return util.normcase(self._root)
-
-    @propertycache
     def _foldmap(self):
         f = {}
         for name in self._map:
@@ -406,7 +402,7 b' class dirstate(object):'
                 folded = path
             else:
                 folded = self._foldmap.setdefault(normed,
-                                util.fspath(normed, self._normroot))
+                                util.fspath(normed, self._root))
         return folded
 
     def normalize(self, path, isknown=False):