git-mergetool: print filename when it contains %

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>