rebase: define options in OPTIONS_SPEC

rebase: define options in OPTIONS_SPEC

Interactive rebase used to have its own command line processing. Since
it used the 'git rev-parse --parseopt' functionality exposed through
git-sh-setup, it had some flexibility, like matching prefixes of long
options, that non-interactive rebase didn't. When interactive rebase's
command line processing was factored out into git-rebase.sh in cf432ca
(rebase: factor out command line option processing, 2011-02-06), this
flexibility was lost. Give back that flexibility to interactive and
non-interactive by defining its options in OPTIONS_SPEC.

Also improve the usage message to contain the --continue, --skip and
--abort sub commands.

Reported-by: Johannes Sixt <redacted>
Signed-off-by: Martin von Zweigbergk <redacted>
Signed-off-by: Junio C Hamano <redacted>