diff: drop unused color reset parameters

diff: drop unused color reset parameters

Several of the emit_* functions take a "reset" color parameter, but
never actually look at it (instead, they call into emit_diff_symbol,
which handles the colors itself). Let's drop these unused parameters.

Note that emit_line() does still take a color/reset pair, and actually
uses it. It cannot be refactored to match these other functions because
it's the thing that emit_diff_symbol eventually calls into (i.e., it
does not by itself know which colors to use, and must be told by the
caller).

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