add-interactive: manually fall back color config to color.ui

Color options like color.interactive and color.diff should fall back to
the value of color.ui if they aren't set. In add-interactive, we check
the specific options (e.g., color.diff) via repo_config_get_value(),
which does not depend on the main command having loaded any color config
via the git_config() callback mechanism.

But then we call want_color() on the result; if our specific config is
unset then that function uses the value of git_use_color_default. That
variable is typically set from color.ui by the git_color_config()
callback, which is called by the main command in its own git_config()
callback function.

This works fine for "add -p", whose add_config() callback calls into
git_color_config(). But it doesn't work for other commands like
"checkout -p", which is otherwise unaware of color at all. People tend
not to notice because the default is "auto", and that's what they'd set
color.ui to as well. But something like:

  git -c color.ui=false checkout -p

should disable color, and it doesn't.

This regression goes back to 0527ccb1b5 (add -i: default to the built-in
implementation, 2021-11-30). In the perl version we got the color config
from "git config --get-colorbool", which did the full lookup for us.

The obvious fix is for git-checkout to add a call to git_color_config()
to its own config callback. But we'd have to do so for every command
with this problem, which is error-prone. Let's see if we can fix it more
centrally.

It is tempting to teach want_color() to look up the value of
repo_config_get_value("color.ui") itself. But I think that would have
disastrous consequences. Plumbing commands, especially older ones, avoid
porcelain config like "color.*" by simply not parsing it in their config
callbacks. Looking up the value of color.ui under the hood would
undermine that.

Instead, let's do that lookup in the add-interactive setup code. We're
already demand-loading other color config there, which is probably fine
(even in a plumbing command like "git reset", the interactive mode is
inherently porcelain-ish). That catches all commands that use the
interactive code, whether they were calling git_color_config()
themselves or not.

Reported-by: Isaac Oscar Gariano <isaacoscar@live.com.au>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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