dirstate: make sure rootdir ends with directory separator (issue4557)

ntpath.join() of Python 2.7.9 does not work as expected if root is a UNC path
to top of share.

This patch doesn't take care of os.altsep, '/' on Windows, because root should
be normalized by realpath().

mercurial/dirstate.py

@@ -38,7 +38,12 b' class dirstate(object):'
         self._opener = opener
         self._validate = validate
         self._root = root
-        self._rootdir = os.path.join(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._dirty = False
         self._dirtypl = False
         self._lastnormaltime = 0