want_color: automatically fallback to color.ui

All of the "do we want color" flags default to -1 to
indicate that we don't have any color configured. This value
is handled in one of two ways:

  1. In porcelain, we check early on whether the value is
     still -1 after reading the config, and set it to the
     value of color.ui (which defaults to 0).

  2. In plumbing, it stays untouched as -1, and want_color
     defaults it to off.

This works fine, but means that every porcelain has to check
and reassign its color flag. Now that want_color gives us a
place to put this check in a single spot, we can do that,
simplifying the calling code.

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

builtin/diff.c

@@ -277,9 +277,6 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 	gitmodules_config();
 	git_config(git_diff_ui_config, NULL);
 
-	if (diff_use_color_default == -1)
-		diff_use_color_default = git_use_color_default;
-
 	init_revisions(&rev, prefix);
 
 	/* If this is a no-index diff, just run it and exit there. */