tag: respect color.ui config

tag: respect color.ui config

Since 11b087adfd (ref-filter: consult want_color() before
emitting colors, 2017-07-13), we expect that setting
"color.ui" to "always" will enable color tag formats even
without a tty.  As that commit was built on top of
136c8c8b8f (color: check color.ui in git_default_config(),
2017-07-13) from the same series, we didn't need to touch
tag's config parsing at all.

However, since we reverted 136c8c8b8f, we now need to
explicitly call git_color_default_config() to make this
work.

Let's do so, and also restore the test dropped in 0c88bf5050
(provide --color option for all ref-filter users,
2017-10-03). That commit swapped out our "color.ui=always"
test for "--color" in preparation for "always" going away.
But since it is here to stay, we should test both cases.

Note that for-each-ref also lost its color.ui support as
part of reverting 136c8c8b8f. But as a plumbing command, it
should _not_ respect the color.ui config. Since it also
gained a --color option in 0c88bf5050, that's the correct
way to ask it for color. We'll continue to test that, and
confirm that "color.ui" is not respected.

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