rebase: stricter check of standalone sub command

rebase: stricter check of standalone sub command

The sub commands '--continue', '--skip' or '--abort' may only be used
standalone according to the documentation. Other options following the
sub command are currently not accepted, but options preceeding them
are. For example, 'git rebase --continue -v' is not accepted, while
'git rebase -v --continue' is. Tighten up the check and allow no other
options when one of these sub commands are used.

Only check that it is standalone for non-interactive rebase for
now. Once the command line processing for interactive rebase has been
replaced by the command line processing in git-rebase.sh, this check
will also apply to interactive rebase.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>