git-rebase: support --whitespace=<option>

Pass --whitespace=<option> to git-apply.  Since git-apply and git-am
expect this, I'm always surprised when I try to give it to git-rebase
and it doesn't work.

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

git-rebase.sh

@@ -216,8 +216,11 @@ do
 	-v|--verbose)
 		verbose=t
 		;;
+	--whitespace=*)
+		git_am_opt="$git_am_opt $1"
+		;;
 	-C*)
-		git_am_opt=$1
+		git_am_opt="$git_am_opt $1"
 		shift
 		;;
 	-*)