rebase -i --root: fix check for number of arguments

rebase -i --root: fix check for number of arguments

If we are not rebasing with --root, then $# can only be either 1 (base)
or 2 (base and the name of the branch to be rebased).

If we are rebasing with --root, then it is Ok if $# is 0 (rebase the
current branch down to everything) or 1 (rebase the named branch down to
everything).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Acked-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>