transaction: avoid ambiguity of file stat at closing transaction

Files below, which might be changed at closing transaction, are used
to examine validity of cached properties. If changing keeps ctime,
mtime and size of a file, change is overlooked, and old contents
cached before change isn't invalidated as expected.

  - .hg/bookmarks
  - .hg/dirstate
  - .hg/phaseroots

To avoid ambiguity of file stat, this patch writes files out with
checkambig=True at closing transaction.

checkambig becomes True only at closing (= 'not suffix'), because stat
information of '.pending' file isn't used to examine validity of
cached properties.

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

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

mercurial/transaction.py

@@ -310,7 +310,8 b' class transaction(object):'
                         self.registertmp(name, location=location)
                     else:
                         self.addbackup(name, location=location)
-                    files.append(vfs(name, 'w', atomictemp=True))
+                    files.append(vfs(name, 'w', atomictemp=True,
+                                     checkambig=not suffix))
                 genfunc(*files)
             finally:
                 for f in files: