vfs: use checkambigatclosing in checkambig=True but atomictemp=False case

In Mercurial source tree, opening a file in "a"/"a+" mode like below
doesn't specify atomictemp=True for vfs, and this avoids file stat
ambiguity check by atomictempfile.

  - writing changes out in revlog layer uses "a+" mode
  - truncation in repair.strip() uses "a" mode
  - truncation in transaction._playback() uses "a" mode

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 revlog-style file (and close transaction)
  2. discard appended data by truncation (strip or rollback)
  3. append same size but different data to revlog-style file again

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

This patch uses checkambigatclosing in checkambig=True but
atomictemp=False case, to check (and get rid of) file stat ambiguity
at closing.

This is a part of ExactCacheValidationPlan.

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

mercurial/scmutil.py

@@ -587,6 +587,12 @@ class vfs(abstractvfs):
         if nlink == 0:
             self._fixfilemode(f)
 
+        if checkambig:
+            if mode in ('r', 'rb'):
+                raise error.Abort(_('implementation error: mode %s is not'
+                                    ' valid for checkambig=True') % mode)
+            fp = checkambigatclosing(fp)
+
         if backgroundclose:
             if not self._backgroundfilecloser:
                 raise error.Abort(_('backgroundclose can only be used when a '