manifest: specify checkambig=True to revlog.__init__, to avoid ambiguity

If steps below occurs at "the same time in sec", all of mtime, ctime
and size are same between (1) and (3).

  1. append data to 00manifest.i (and close transaction)
  2. discard appended data by truncation (strip or rollback)
  3. append same size but different data to 00manifest.i again

Therefore, cache validation doesn't work after (3) as expected.

To avoid such file stat ambiguity around truncation, this patch
specifies checkambig=True to revlog.__init__(). This makes revlog
write changes out with checkambig=True.

Even after this patch, avoiding file stat ambiguity of 00manifest.i
around truncation isn't yet completed, because truncation side isn't
aware of this issue.

This is a part of ExactCacheValidationPlan.

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

mercurial/manifest.py

@@ -927,7 +927,8 b' class manifestrevlog(revlog.revlog):'
         else:
             self._dirlogcache = {'': self}
 
-        super(manifestrevlog, self).__init__(opener, indexfile)
+        super(manifestrevlog, self).__init__(opener, indexfile,
+                                             checkambig=bool(dir))
 
     @property
     def fulltextcache(self):