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