Fix bash completion with git 2.30.0 and higher

[cblecker]

This implements the fix proposed by @ches here: https://github.com/github/hub/issues/2684#issuecomment-754664270

I've been running this in my local env for over a year without issue, but when installing hub on a new machine, it messed up my bash completions for git, so I thought I'd actually open this PR.

fixes #2684

[cblecker]

I've done basic testing on 2.29.2 and it works, but haven't done a complete regression test or anything.

[cblecker]

(P.S. If it's possible for a version to be cut with this and https://github.com/github/hub/pull/2939, it would be much appreciated)

[cblecker]

@mislav: friendly nudge :) is getting this merged and a release cut possible? Thank you!

[kbrock]

Thanks for putting this PR together

I verified it works on my system. (red green tested) When installing but git is not installed, autocomplete didn't work so well. But I'm assuming that git needs to be installed in that case

I put other notes in the main issue

==> bash-completion@2: stable 2.11 (bottled), HEAD
==> git: stable 2.41.0 (bottled), HEAD
==> hub: stable 2.14.2 (bottled), HEAD