config: refactor get_colorbool function

config: refactor get_colorbool function

For "git config --get-colorbool color.foo", we use a custom
callback that looks not only for the key that the user gave
us, but also for "diff.color" (for backwards compatibility)
and "color.ui" (as a fallback).

For the former, we use a custom variable to store the
diff.color value. For the latter, though, we store it in the
main "git_use_color_default" variable, turning on color.ui
for any other parts of git that respect this value.

In practice, this doesn't cause any bugs, because git-config
runs without caring about git_use_color_default, and then
exits. But it crosses module boundaries in an unusual and
confusing way, and it makes refactoring color handling
harder than it needs to be.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>