tests/test-revert-flags.t

hghave: add a check for the `xz` compression utility

This isn't install by default on Mac, which causes a test failure.  The logic
for avoiding the command is a little goofy, but nested `#if` isn't supported,
and it still seems worth running the hg command to see if anything explodes.

With this, the py3 tests run (almost) cleanly on 10.14.6:

    # Ran 835 tests, 58 skipped, 1 failed.

Alas, the mac-packaging test is skipped because it's slow.  The failure here is
in test-releasenotes-merging.t, complaining about not being able to import the
`fuzzywuzzy` module.  I have it installed on py3 (thus the test isn't skipped),
but not on py2.  So there must be some unintended cross pollination here when
running `hg` commands.

Differential Revision: https://phab.mercurial-scm.org/D7374

#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 ..