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>

git-rebase--interactive.sh

@@ -571,7 +571,8 @@ first and then run 'git rebase --continue' again."
 		;;
 	--)
 		shift
-		test ! -z "$REBASE_ROOT" -o $# -eq 1 -o $# -eq 2 || usage
+		test -z "$REBASE_ROOT" -a $# -ge 1 -a $# -le 2 ||
+		test ! -z "$REBASE_ROOT" -a $# -le 1 || usage
 		test -d "$DOTEST" &&
 			die "Interactive rebase already started"