completion: teach rebase to use __gitcomp_builtin

completion: teach rebase to use __gitcomp_builtin

Currently, _git_rebase() uses a hardcoded list of options for its
completion. However, we can use __gitcomp_builtin() to get a dynamically
generated list of completions instead.

Teach _git_rebase() to use __gitcomp_builtin() so that newly implemented
options in rebase will be automatically completed without any mucking
around in git-completion.bash.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>