diff: pass o->use_color directly to fill_metainfo()

We pass the use_color parameter of fill_metainfo() as a strict boolean,
using:

  want_color(o->use_color) && !pgm

to derive its value. But then inside the function, we pass it to
diff_get_color(), which expects one of the git_colorbool enum values,
and so feeds it to want_color() again.

Even though want_color() produces a strict 0/1 boolean, this doesn't
produce wrong results because want_color() is idempotent. Since
GIT_COLOR_ALWAYS and NEVER are defined as 1 and 0, and because
want_color() passes through those values, evaluating "want_color(foo)"
and "want_color(want_color(foo))" will return the same result.

But as part of a longer strategy to align the types we use for storing
these values, let's pass through the colorbool directly. To handle the
"&&" case here, we'll convert the presence of "pgm" into "NEVER", which
arguably makes the intent of the code more clear anyway.

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

diff.c

@@ -4580,7 +4580,7 @@ static void run_diff_cmd(const struct external_diff *pgm,
 		 */
 		fill_metainfo(msg, name, other, one, two, o, p,
 			      &must_show_header,
-			      want_color(o->use_color) && !pgm);
+			      pgm ? GIT_COLOR_NEVER : o->use_color);
 		xfrm_msg = msg->len ? msg->buf : NULL;
 	}