p4merge: 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>

mergetools/p4merge

@@ -30,5 +30,5 @@ create_empty_file () {
 	empty_file="${TMPDIR:-/tmp}/git-difftool-p4merge-empty-file.$$"
 	>"$empty_file"
 
-	printf "$empty_file"
+	printf "%s" "$empty_file"
 }