rebase -m: cleanup --strategy-option handling

rebase -m: cleanup --strategy-option handling

When handling "--strategy-option" rebase collects the commands into a
struct string_list, then concatenates them into a string, prepending "--"
to each one before splitting the string and removing the "--" prefix.
This is an artifact of the scripted rebase and the need to support
"rebase --preserve-merges". Now that "--preserve-merges" no-longer
exists we can cleanup the way the argument is handled.

The tests for a bad strategy option are adjusted now that
parse_strategy_opts() is no-longer called when starting a rebase. The
fact that it only errors out when running "git rebase --continue" is a
mixed blessing but the next commit will fix the root cause of the
parsing problem so lets not worry about that here.

Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>