color: check color.ui in git_default_config()

color: check color.ui in git_default_config()

Back in prehistoric times, our decision on whether or not to
show color by default relied on using a config callback that
either did or didn't load color config like color.diff.
When we introduced color.ui, we put it in the same boat:
commands had to manually respect it by using git_color_config()
or its git_color_default_config() convenience wrapper.

But in 4c7f1819b (make color.ui default to 'auto',
2013-06-10), that changed. Since then, we default color.ui
to auto in all programs, meaning that even plumbing commands
like "git diff-tree --pretty" might colorize the output.
Nobody seems to have complained in the intervening years,
presumably because the "is stdout a tty" check does a good
job of catching the right cases.

But that leaves an interesting curiosity: color.ui defaults
to auto even in plumbing, but you can't actually _disable_
the color via config. So if you really hate color and set
"color.ui" to false, diff-tree will still show color (but
porcelain like git-diff won't).  Nobody noticed that either,
probably because very few people disable color.

One could argue that the plumbing should _always_ disable
color unless an explicit --color option is given on the
command line. But in practice, this creates a lot of
complications for scripts which do want plumbing to show
user-visible output. They can't just pass "--color" blindly;
they need to check the user's config and decide what to
send.

Given that nobody has complained about the current behavior,
let's assume it's a good path, and follow it to its
conclusion: supporting color.ui everywhere.

Note that you can create havoc by setting color.ui=always in
your config, but that's more or less already the case. We
could disallow it entirely, but it is handy for one-offs
like:

  git -c color.ui=always foo >not-a-tty

when "foo" does not take a --color option itself.

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