completion: learn to complete `git rebase --onto=`

completion: learn to complete `git rebase --onto=`

In 2b9bd488ae ("completion: teach rebase to use __gitcomp_builtin",
2019-09-12), the completion script learned to complete rebase using
__gitcomp_builtin(). However, this resulted in `--onto=` being suggested
instead of `--onto `.

Before, when there was a space, we'd start a new word and, as a result,
fallback to __git_complete_refs() and `--onto` would be completed this
way. However, now we match the `--*` case which does not know how to
offer completions for refs.

Teach _git_rebase() to complete refs in the `--onto=` case so that we
fix this regression.

Reported-by: Paul Jolly <paul@myitcv.io>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>