Revert "color: make "always" the same as "auto" in config"

Revert "color: make "always" the same as "auto" in config"

This reverts commit 6be4595edb8e5b616c6e8b9fbc78b0f831fa2a87.

That commit weakened the "always" setting of color config so
that it acted as "auto". This was meant to solve regressions
in v2.14.2 in which setting "color.ui=always" in the on-disk
config broke scripts like add--interactive, because the
plumbing diff commands began to generate color output.

This was due to 136c8c8b8f (color: check color.ui in
git_default_config(), 2017-07-13), which was in turn trying
to fix issues caused by 4c7f1819b3 (make color.ui default to
'auto', 2013-06-10). But in weakening "always", we created
even more problems, as people expect to be able to use "git
-c color.ui=always" to force color (especially because some
commands don't have their own --color flag). We can fix that
by special-casing the command-line "-c", but now things are
getting pretty confusing.

Instead of piling hacks upon hacks, let's start peeling off
the hacks. The first step is dropping the weakening of
"always", which this revert does.

Note that we could actually revert the whole series merged
in by da15b78e52642bd45fd5513ab0000fdf2e58a6f4. Most of that
series consists of preparations to the tests to handle the
weakening of "-c color.ui=always". But it's worth keeping
for a few reasons:

  - there are some other preparatory cleanups, like
    e433749d86 (test-terminal: set TERM=vt100, 2017-10-03)

  - it adds "--color" options more consistently in
    0c88bf5050 (provide --color option for all ref-filter
    users, 2017-10-03)

  - some of the cases dropping "-c" end up being more robust
    and realistic tests, as in 01c94e9001 (t7508: use
    test_terminal for color output, 2017-10-03)

  - the preferred tool for overriding config is "--color",
    and we should be modeling that consistently

We can individually revert the few commits necessary to
restore some useful tests (which will be done on top of this
patch).

Note that this isn't a pure revert; we'll keep the test
added in t3701, but mark it as failure for now.

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