rebase --rebase-merges: root commits can be cousins, too

Reported by Wink Saville: when rebasing with no-rebase-cousins, we
will want to refrain from rebasing all of them, even when they are
root commits.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -3903,7 +3903,8 @@ static int make_script_with_merges(struct pretty_print_context *pp,
 		}
 
 		if (!commit)
-			fprintf(out, "%s onto\n", cmd_reset);
+			fprintf(out, "%s %s\n", cmd_reset,
+				rebase_cousins ? "onto" : "[new root]");
 		else {
 			const char *to = NULL;