tests/test-revert-flags.t

upgrade: remove unnecessary `is None` check

`upgrade_engine.upgrade()` always return the `backuppath` value and there are
not early returns. Hence I don't see how `backuppath` can be None.

Adding extra unncessary safe checks hides unknown bugs. Hence removing it.

Differential Revision: https://phab.mercurial-scm.org/D9675

#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 ..