provide --color option for all ref-filter users

When ref-filter learned about want_color() in 11b087adfd
(ref-filter: consult want_color() before emitting colors,
2017-07-13), it became useful to be able to turn colors off
and on for specific commands. For git-branch, you can do so
with --color/--no-color.

But for git-for-each-ref and git-tag, the other users of
ref-filter, you have no option except to tweak the
"color.ui" config setting. Let's give both of these commands
the usual color command-line options.

This is a bit more obvious as a method for overriding the
config. And it also prepares us for the behavior of "always"
changing (so that we are still left with a way of forcing
color when our output goes to a non-terminal).

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

builtin/tag.c

@@ -411,6 +411,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		},
 		OPT_STRING(  0 , "format", &format.format, N_("format"),
 			   N_("format to use for the output")),
+		OPT__COLOR(&format.use_color, N_("respect format colors")),
 		OPT_BOOL('i', "ignore-case", &icase, N_("sorting and filtering are case insensitive")),
 		OPT_END()
 	};