phases: make writing phaseroots file out avoid ambiguity of file stat

Cached attribute repo._phasecache uses stat of '.hg/phaseroots' file
to examine validity of cached contents. If writing '.hg/phaseroots'
file out keeps ctime, mtime and size of it, change is overlooked, and
old contents cached before change isn't invalidated as expected.

To avoid ambiguity of file stat, this patch writes '.hg/phaseroots'
file out with checkambig=True.

This patch is a part of "Exact Cache Validation Plan":

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

mercurial/phases.py

@@ -251,7 +251,7 b' class phasecache(object):'
     def write(self):
         if not self.dirty:
             return
-        f = self.opener('phaseroots', 'w', atomictemp=True)
+        f = self.opener('phaseroots', 'w', atomictemp=True, checkambig=True)
         try:
             self._write(f)
         finally: