scmutil: ignore EPERM at os.utime, which avoids ambiguity at closing

According to POSIX specification, just having group write access to a
file causes EPERM at invocation of os.utime() with an explicit time
information (e.g. working on the repository shared by group access
permission).

To ignore EPERM at closing file object in such case, this patch makes
checkambigatclosing._checkambig() use filestat.avoidambig() introduced
by previous patch.

Some functions below imply this code path at truncation of an existing
(= might be owned by another user) file.

  - strip() in repair.py, introduced by e38d85be978f
  - _playback() in transaction.py, introduced by 599912a62ff6

This is a variant of issue5418.

mercurial/scmutil.py

@@ -1457,8 +1457,7 b' class checkambigatclosing(closewrapbase)'
             newstat = util.filestat(self._origfh.name)
             if newstat.isambig(oldstat):
                 # stat of changed file is ambiguous to original one
-                advanced = (oldstat.stat.st_mtime + 1) & 0x7fffffff
-                os.utime(self._origfh.name, (advanced, advanced))
+                newstat.avoidambig(self._origfh.name, oldstat)
 
     def __exit__(self, exc_type, exc_value, exc_tb):
         self._origfh.__exit__(exc_type, exc_value, exc_tb)