am: pass exclude down to apply

This allows to pass patches around from repositories,
where the other repository doesn't feature certain files.

In the special case this works for dash git sync to klibc dash:
 git am --directory="usr/dash" --exclude="usr/dash/configure.ac" \
        --exclude="usr/dash/ChangeLog" --exclude="usr/dash/dash.1" \
	.. -i -s -k ../dash/000X-foo.patch

Signed-off-by: maximilian attems <max@stro.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -22,6 +22,7 @@ whitespace=     pass it through git-apply
 ignore-space-change pass it through git-apply
 ignore-whitespace pass it through git-apply
 directory=      pass it through git-apply
+exclude=        pass it through git-apply
 C=              pass it through git-apply
 p=              pass it through git-apply
 patch-format=   format the patch(es) are in
@@ -362,7 +363,7 @@ do
 		;;
 	--resolvemsg)
 		shift; resolvemsg=$1 ;;
-	--whitespace|--directory)
+	--whitespace|--directory|--exclude)
 		git_apply_opt="$git_apply_opt $(sq "$1=$2")"; shift ;;
 	-C|-p)
 		git_apply_opt="$git_apply_opt $(sq "$1$2")"; shift ;;