bash-completion: initial import based on CrOS version

We've had a limited version of this in CrOS for a long time.  There's
nothing CrOS specific about it, so lets move it to the repo project so
everyone can utilize it.

Change-Id: I04cd94610c1100f3afcd2baf8c8e7ab13e589490
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299202


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>