diff --color: use $GIT_DIR/config

diff --color: use $GIT_DIR/config

This lets you use something like this in your $GIT_DIR/config
file.

	[diff]
		color = auto

	[diff.color]
		new = blue
		old = yellow
		frag = reverse

When diff.color is set to "auto", colored diff is enabled when
the standard output is the terminal.  Other choices are "always",
and "never".  Usual boolean true/false can also be used.

The colormap entries can specify colors for the following slots:

	plain	- lines that appear in both old and new file (context)
	meta	- diff --git header and extended git diff headers
	frag	- @@ -n,m +l,k @@ lines (hunk header)
	old	- lines deleted from old file
	new	- lines added to new file

The following color names can be used:

	normal, bold, dim, l, blink, reverse, reset,
	black, red, green, yellow, blue, magenta, cyan,
	white

Signed-off-by: Junio C Hamano <junkio@cox.net>