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>