rebase: cleanup "--exec" option handling

rebase: cleanup "--exec" option handling

When handling "--exec" rebase collects the commands into a struct
string_list, then prepends "exec " to each command creating a multi line
string and finally splits that string back into a list of commands. 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. There is no need to add the
"exec " prefix to the commands as that is added by todo_list_to_strbuf().

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