pretty: use format_commit_context.auto_color as colorbool

When we see "%C(auto)" as a format placeholder, we evaluate the "color"
field of our pretty_print_context to decide whether we want color. The
auto_color field of format_commit_context then stores the boolean result
of want_color(), telling us the yes/no of whether we want color.

But the resulting field is passed to various functions which expect a
git_colorbool, like diff_get_color(), that will then pass it to
want_color() again. It's not wrong to do so, since want_color() is
idempotent. But it makes it harder to reason about the types, since we
sometimes confuse colorbools and strict booleans.

Let's instead store auto_color as the original colorbool itself. We'll
have to make sure it is passed through want_color() when it is
evaluated, but there is only one such spot (right next to where we
assign it!). Every other caller just ends up passing it to get
diff_get_color() either directly or through another helper.

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