git-rebase: fix -C option

The extra shift here causes failure to parse any commandline including
the -C option.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -221,7 +221,6 @@ do
 		;;
 	-C*)
 		git_am_opt="$git_am_opt $1"
-		shift
 		;;
 	-*)
 		usage