dirstate: make restoring from backup avoid ambiguity of file stat

File .hg/dirstate is restored by renaming from backup in failure
inside scopes below. If renaming keeps ctime, mtime and size of a
file, restoring is overlooked, and old contents cached before
restoring isn't invalidated as expected.

  - dirstateguard scope (from '.hg/dirstate.SUFFIX')
  - transaction scope (from '.hg/journal.dirstate')

To avoid ambiguity of file stat at restoring, this patch invokes
vfs.rename() with checkambig=True.

This patch is a part of "Exact Cache Validation Plan":

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

mercurial/dirstate.py

@@ -1243,7 +1243,8 b' class dirstate(object):'
         self.invalidate()
         filename = self._actualfilename(tr)
         # using self._filename to avoid having "pending" in the backup filename
-        self._opener.rename(prefix + self._filename + suffix, filename)
+        self._opener.rename(prefix + self._filename + suffix, filename,
+                            checkambig=True)
 
     def clearbackup(self, tr, suffix='', prefix=''):
         '''Clear backup file with suffix'''