difftool--helper: fix printf usage
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 <368b482b6f12dc9c0b644e97db80e2fd9a8eba38@asheesh.org>
Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git