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

tests/hghave.py

@@ -673,6 +673,13 b' def has_tic():'
         return False
 
 
+@check("xz", "xz compression utility")
+def has_xz():
+    # When Windows invokes a subprocess in shell mode, it uses `cmd.exe`, which
+    # only knows `where`, not `which`.  So invoke MSYS shell explicitly.
+    return matchoutput("sh -c 'test -x \"`which xz`\"'", b'')
+
+
 @check("msys", "Windows with MSYS")
 def has_msys():
     return os.getenv('MSYSTEM')

tests/test-archive.t

@@ -574,8 +574,8 b' test xz support only available in Python'
 
 #if py3
   $ hg archive ../archive.txz
-  $ xz -l ../archive.txz | head -n1
-  Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename
+  $ which xz >/dev/null && xz -l ../archive.txz | head -n1 || true
+  Strms  Blocks   Compressed Uncompressed  Ratio  Check   Filename (xz !)
   $ rm -f ../archive.txz
 #else
   $ hg archive ../archive.txz