that prompts if upstream remote branch(es) should be deleted too. As
with `git branch`, the function supports multiple branches as params,
and also the `-r`/`--remotes` param. Update `gbx` and `gbX` to use this
function.
Also update `git-stash-clear-interactive` to have multiple tests instead
of multiple nested `if`/`then`/`fi`. (Simpler syntax, IMHO)
