config: store want_color() result in a separate bool
The "git config --get-colorbool foo.bar" command not only digs in the
config to find the value of foo.bar, it evaluates the result using
want_color() to check the tty-ness of stdout.
But it stores the bool result of want_color() in the same git_colorbool
that we found in the config. This works in practice because the
git_colorbool enum is a superset of the bool values. But it is an oddity
from a type system perspective.
Let's instead store the result in a separate bool and use that.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>