Fix bash completion on chained command arguments

[stopthatcow]

Enables bash completion on chained command arguments, allows completions with spaces in them, makes bash completion about 10ms faster by exiting python using os._exit() rather than sys.exit(), and updates documentation that was incorrect from PR #755

Fixes#754
Fixes#769

[stopthatcow]

@Gig77 Feel free to see if this solves your issue.

[Gig77]

It does, thanks a lot!

[stopthatcow]

@untitaker I'd love to get your thoughts on this.

[untitaker]

A lot of unrelated things are in this PR, I'm unsure if we want fast_exit. It potentially makes this a pain to test.

[stopthatcow]

@untitaker good call. I broke this into #772, #773, #774, and #775. I think fast_exit in this specific case is ok because it is only used when bash completion is invoked (something we don't test directly). If we did want to test it we'd just need to launch the test using subprocess. Bzr actually uses this 'trick' in all exit code paths I believe to try to be more responsive as a CLI. I got the inspiration from here.

[untitaker]

we'd just need to launch the test using subprocess.

not a huge fan of this, but we'll see when we get to test it. I was thinking more about monkeypatching fast_exit.