add basic branch-completion

[nammn]

Missing

• check whether we want to remove the autogenerated commandline options,
• support for track

Description of change

adds basic branch completion

QA steps

source $GOPATH/src/github.com/juju/juju/etc/bash_completion.d
juju branch

~/golang/src/github.com/juju/juju/etc/bash_completion.d branch-completion*
❯ juju branch <tab>
bla                 -h                  --logging-config    --model             --quiet             test                --verbose                                                                 
--debug             --help              -m                  --no-browser-login  --show-log          testest                                                                                   

~/golang/src/github.com/juju/juju/etc/bash_completion.d branch-completion*
❯ juju branch tes <tab>
test     teste

same as above for
juju abort
juju add-branch

juju track

06:13:50 nam@nams-canon bash_completion.d ±|branch-completion ✗|→ juju track 
bla                 -h                  --logging-config    --model             --quiet             test                ubuntu              ubuntu/1            --verbose           
--debug             --help              -m                  --no-browser-login  --show-log          testest             ubuntu/0            ubuntu/2     

[SimonRichardson]

$$merge$$