changelog: specify checkambig=True to avoid ambiguity around truncation

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 00changelog.i (and close transaction)
  2. discard appended data by truncation (strip or rollback)
  3. append same size but different data to 00changelog.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 for renaming or opening to write changes out
at finalization.

Even after this patch, avoiding file stat ambiguity of 00changelog.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/changelog.py

@@ -382,9 +382,9 b' class changelog(revlog.revlog):'
             tmpname = self.indexfile + ".a"
             nfile = self.opener.open(tmpname)
             nfile.close()
-            self.opener.rename(tmpname, self.indexfile)
+            self.opener.rename(tmpname, self.indexfile, checkambig=True)
         elif self._delaybuf:
-            fp = self.opener(self.indexfile, 'a')
+            fp = self.opener(self.indexfile, 'a', checkambig=True)
             fp.write("".join(self._delaybuf))
             fp.close()
             self._delaybuf = None