rebase: remove useless arguments check

Remove a check on the number of arguments for --onto and -x options.
It is not possible for $# to be <= 2 at this point :

 - if --onto or -x has an argument, git rev-parse --parseopt will
   provide something like this :
     set -- --onto 'x' --
   when parsing the "--onto" option, $# will be 3 or more if there are
   other options.

 - if --onto or -x doesn't have an argument, git rev-parse --parseopt
   will exit with an error and display usage information.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
rebase: don't try to match -M option

The -M option does not exist in OPTIONS_SPEC, so there is no use to try
to find it.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
rebase: parse options in stuck-long mode

There is no functionnal change. The reason for this change is to be able
to add a new option taking an optional argument.

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>
rebase: add the --gpg-sign option

Signed-off-by: Nicolas Vigier <boklm@mars-attacks.org>