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

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extra : moz-landing-system : lando