color.c: refactor color_output arguments

color.c: refactor color_output arguments

color_output() takes a "type" parameter, which is either '3' or '4',
and that byte is shown in front of '0'-'7' to form "30"-"37" or
"40"-"47" in ANSI output mode for fore-ground and back-ground
colors.

Clarify the purpose of the parameter by renaming it to the
"background" that is a boolean.

Also, change the .value field in the color struct from storing 0-7
for basic 8 colors to storing 30-37 for ANSI colors.  This aligns
the code to show ANSI colors to the code for the 256 color scheme,
which already uses the actual value to be sent to the terminal.

Signed-off-by: Eyal Soha <shawarmakarma@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>