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

changelog: 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 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 to revlog.__init__(). This makes revlog
write changes out with checkambig=True.

Even though changes of 00changelog.i themselves are written out at
changelog._finalize(), this checkambig=True is needed, because
revlog.checkinlinesize(), which is invoked at the end of
changelog._finalize(), might replace already changed 00changelog.i by
converted one.

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

--- a/mercurial/changelog.py	Thu Sep 22 21:51:59 2016 +0900
+++ b/mercurial/changelog.py	Thu Sep 22 21:51:59 2016 +0900
@@ -257,7 +257,8 @@
 
 class changelog(revlog.revlog):
     def __init__(self, opener):
-        revlog.revlog.__init__(self, opener, "00changelog.i")
+        revlog.revlog.__init__(self, opener, "00changelog.i",
+                               checkambig=True)
         if self._initempty:
             # changelogs don't benefit from generaldelta
             self.version &= ~revlog.REVLOGGENERALDELTA