difftool--helper: fix printf usage

Do not use a random string as if it is a format string for printf
when showing it literally; instead feed it to '%s' format.

Reported-by: Asheesh Laroia <asheesh@asheesh.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -40,7 +40,7 @@ launch_merge_tool () {
 	# the user with the real $MERGED name before launching $merge_tool.
 	if should_prompt
 	then
-		printf "\nViewing: '$MERGED'\n"
+		printf "\nViewing: '%s'\n" "$MERGED"
 		if use_ext_cmd
 		then
 			printf "Launch '%s' [Y/n]: " \