This adds an option to help scripts find out color settings from
the configuration file.

    git config --get-colorbool color.diff

inspects color.diff variable, and exits with status 0 (i.e. success) if
color is to be used.  It exits with status 1 otherwise.

If a script wants "true"/"false" answer to the standard output of the
command, it can pass an additional boolean parameter to its command
line, telling if its standard output is a terminal, like this:

    git config --get-colorbool color.diff true

When called like this, the command outputs "true" to its standard output
if color is to be used (i.e. "color.diff" says "always", "auto", or
"true"), and "false" otherwise.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>