Since we're calling into a mach command for the current completion
implementation anyway (and incurring python startup penalties), we
might as well move all the bash logic into the mach command.
The new 'mach-completion' command was created in case there are
scripts relying on the current behaviour of 'mach-commands'.
Depends on D16254
Differential Revision: https://phabricator.services.mozilla.com/D16255
--HG--
extra : moz-landing-system : lando