dirstate: use pathutil.normasprefix to ensure os.sep at the end of root stable

dirstate: use pathutil.normasprefix to ensure os.sep at the end of root

3cc630be5f09 replaced "os.path.join(root, '')" by
"root.endswith(os.sep)" examination, because Python 2.7.9 changes
behavior of "os.path.join(path, '')" on UNC path.

But some problematic encodings use 0x5c (= "os.sep" on Windows) as the
tail byte of some multi-byte characters, and replacement above
prevents Mercurial from working on the repository, of which root path
ends with such multi-byte character, regardless of enabling win32mbcs.

This patch uses "pathutil.normasprefix()" instead of
"root.endswith(os.sep)" examination, to ensure "os.sep" at the end of
"dirstate._rootdir" even with problematic encodings.

"root" of dirstate can be passed to "pathutil.normasprefix()" without
normalization, because it is always given from "repo.root" =
"repo.wvfs.base", which is normalized by "os.path.realpath()".

Using "util.endswithsep()" instead of "str.endswith(os.sep)" also
fixes this problem, but this patch chooses "pathutil.normasprefix()"
to centralize "adding os.sep if endswith(os.sep)" logic into it.

--- a/mercurial/dirstate.py	Tue Apr 21 21:02:08 2015 -0300
+++ b/mercurial/dirstate.py	Wed Apr 22 23:38:52 2015 +0900
@@ -40,10 +40,7 @@
         self._root = root
         # ntpath.join(root, '') of Python 2.7.9 does not add sep if root is
         # UNC path pointing to root share (issue4557)
-        if root.endswith(os.sep):
-            self._rootdir = root
-        else:
-            self._rootdir = root + os.sep
+        self._rootdir = pathutil.normasprefix(root)
         self._dirty = False
         self._dirtypl = False
         self._lastnormaltime = 0