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> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>