grep: Colorize selected, context, and function lines

grep: Colorize selected, context, and function lines

Colorize non-matching text of selected lines, context lines, and
function name lines.  The default for all three is no color, but they
can be configured using color.grep.<slot>.  The first two are similar
to the corresponding options in GNU grep, except that GNU grep applies
the color to the entire line, not just non-matching text.

Signed-off-by: Mark Lodato <redacted>
Signed-off-by: Junio C Hamano <redacted>