dirstate: make writing branch file out avoid ambiguity of file stat

Cached attribute dirstate._branch uses stat of '.hg/branch' file to
examine validity of cached contents. If writing '.hg/branch' 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/branch' file
out with checkambig=True.

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

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

mercurial/dirstate.py

@@ -367,7 +367,7 b' class dirstate(object):'
 
     def setbranch(self, branch):
         self._branch = encoding.fromlocal(branch)
-        f = self._opener('branch', 'w', atomictemp=True)
+        f = self._opener('branch', 'w', atomictemp=True, checkambig=True)
         try:
             f.write(self._branch + '\n')
             f.close()