diff: refactor COLOR_DIFF from a flag into an int

This lets us store more than just a bit flag for whether we
want color; we can also store whether we want automatic
colors. This can be useful for making the automatic-color
decision closer to the point of use.

This mostly just involves replacing DIFF_OPT_* calls with
manipulations of the flag. The biggest exception is that
calls to DIFF_OPT_TST must check for "o->use_color > 0",
which lets an "unknown" value (i.e., the default) stay at
"no color". In the previous code, a value of "-1" was not
propagated at all.

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

wt-status.c

@@ -681,7 +681,7 @@ static void wt_status_print_verbose(struct wt_status *s)
 	 * will have checked isatty on stdout).
 	 */
 	if (s->fp != stdout)
-		DIFF_OPT_CLR(&rev.diffopt, COLOR_DIFF);
+		rev.diffopt.use_color = 0;
 	run_diff_index(&rev, 1);
 }