diff: rename "set" variable

diff: rename "set" variable

Once upon a time the builtin_diff function used one color, and the color
variables were called "set" and "reset". Nowadays it is a much longer
function and we use several colors (e.g., "add", "del"). Rename "set" to
"meta" to show that it is the color for showing diff meta-info (it still
does not indicate that it is a "color", but at least it matches the
scheme of the other color variables).

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