color: return enum from git_config_colorbool()

The git_config_colorbool() function returns an integer which is always
one of the GIT_COLOR_* constants UNKNOWN, NEVER, ALWAYS, or AUTO. We
define these constants with macros, but let's switch to using an enum.
Even though the compiler does not strictly enforce enum/int conversions,
this should make the intent clearer to human readers. And as a bonus,
enum names are typically available to debuggers, making it more pleasant
to step through the code there.

This patch updates the return type of git_config_colorbool(), but holds
off on updating all of the callers. There's some trickiness to some of
them, and in the meantime it's perfectly fine to assign an enum into an
int.

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