tests/test-revert-flags.t

context: stop setting None for modified or added nodes

Instead use a magic value, so that we can identify modified or added
nodes correctly when using manifest.diff().

Thanks to Martin von Zweigbergk for catching that we have to update
_buildstatus as well. That part eluded my debugging for some time.

#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 ..