transaction: avoid ambiguity of file stat at restoring from backup

In some cases below, copying from backup is used to restore original
contents of a file, which is backuped via addfilegenerator(). If
copying keeps ctime, mtime and size of a file, restoring is
overlooked, and old contents cached before restoring isn't invalidated
as expected.

  - failure of transaction (from '.hg/journal.backup.*')
  - rollback of previous transaction (from '.hg/undo.backup.*')

To avoid ambiguity of file stat at restoring, this patch invokes
util.copyfile() with checkambig=True.

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

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

mercurial/transaction.py

@@ -72,7 +72,7 b' def _playback(journal, report, opener, v'
                 filepath = vfs.join(f)
                 backuppath = vfs.join(b)
                 try:
-                    util.copyfile(backuppath, filepath)
+                    util.copyfile(backuppath, filepath, checkambig=True)
                     backupfiles.append(b)
                 except IOError:
                     report(_("failed to recover %s\n") % f)