Bug 1242678 - Disable mozext when running hg commands; r=dminor

We disable various extensions when running `hg` commands as part
of `mach mercurial-setup` because they can interfere with operations.
e.g. if the extension isn't compatible with your version of hg, you
will get an error.

For some reason "mozext" wasn't part of this list. Fix that.

DONTBUILD (NPOTB)

tools/mercurial/hgsetup/update.py

@@ -53,6 +53,7 @@ class MercurialUpdater(object):
             'bzpost',
             'firefoxtree',
             'hgwatchman',
+            'mozext',
             'mqext',
             'qimportbz',
             'push-to-try',