Fast tab completion for mach

[Andrew Halberstadt]

I'm pleased to announce fast mach tab completion for bash, zsh and fish shells! Follow the directions here to get set up:

https://firefox-source-docs.mozilla.org/mach/usage.html#tab-completion

These scripts have a couple big advantages over the old completion script:

1. They are much faster (no more 1-2s delay on tab)

2. They display the help text of commands / arguments while completing (zsh and fish only).

The old completion script will still hang around for now, but it should be considered deprecated and could be removed at a later date.

The only drawback of the new scripts is that they need to be periodically re-generated as commands and arguments are hardcoded into the completion script itself. This can be pretty easily accomplished via a cron task. Bug 1670288 also tracks (re)generating + installing these scripts as part of `mach bootstrap` (I don't have time / plans to work on this anytime soon however).

Please let me know if you have any problems getting it set up. Bugs can be filed under Firefox Build System :: Mach Core.

Cheers,

Andrew