difftool: hack around -Wzero-length-format warning

difftool: hack around -Wzero-length-format warning

Building with "gcc -Wall" will complain that the format in:

  warning("")

is empty. Which is true, but the warning is over-eager. We
are calling the function for its side effect of printing
"warning:", even with an empty string.

Our DEVELOPER Makefile knob disables the warning, but not
everybody uses it. Let's silence the warning in the code so
that nobody reports it or tries to "fix" it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>