git-mergetool: print filename when it contains %

If git-mergetool was invoked with files with a percent sign (%) in their names, it would print an error. For example, if you were calling mergetool on a file called "%2F": printf: %2F: invalid directive Do not pass random string to printf as if it were a valid format. Use format string "%s" and pass the string as data to be formatted instead. Signed-off-by: Asheesh Laroia <asheesh@asheesh.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>