rebase --root: print usage on too many args

rebase --root: print usage on too many args

Just like

  git rebase --onto newbase upstream branch error

displays the usage message, so should clearly

  git rebase --onto newbase --root branch error

, but it doesn't. Instead, it ignores both "branch" and "error" and
rebases the current HEAD. This is because we try to match the number
of remainging arguments "$#", which fails to match "1" argument and
matches the "*" that really should have been a "0".

Make sure we display usage information when too many arguments are
given. Also fail-fast in case of similar bugs in the future by
matching on exactly 0 arguments and failing on unknown numbers.

Signed-off-by: Junio C Hamano <gitster@pobox.com>