tests/test-revert-flags.t

tests: replace mockmakedate function in test-amend.t

This is a follow up patch for D5554. This replaces mockmakedate function
by `tests/mockmakedate.py`.

Differential Revision: https://phab.mercurial-scm.org/D5577

#require execbit

  $ hg init repo
  $ cd repo
  $ echo foo > foo
  $ chmod 644 foo
  $ hg ci -qAm '644'

  $ chmod 755 foo
  $ hg ci -qAm '755'

reverting to rev 0

  $ hg revert -a -r 0
  reverting foo
  $ hg st
  M foo
  $ hg diff --git
  diff --git a/foo b/foo
  old mode 100755
  new mode 100644

  $ cd ..