tests/test-empty-dir.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

  $ hg init

  $ echo 123 > a
  $ hg add a
  $ hg commit -m "first" a

  $ mkdir sub
  $ echo 321 > sub/b
  $ hg add sub/b
  $ hg commit -m "second" sub/b

  $ cat sub/b
  321

  $ hg co 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ cat sub/b 2>/dev/null || echo "sub/b not present"
  sub/b not present

  $ test -d sub || echo "sub not present"
  sub not present