add: check color.ui for interactive add

add: check color.ui for interactive add

When 'git add -i' and 'git add -p' were converted to a builtin, they
introduced a color bug: the 'color.ui' config setting is ignored.

The included test demonstrates an example that is similar to the
previous test, which focuses on customizing colors. Here, we are
demonstrating that colors are not being used at all by comparing the raw
output and the color-decoded version of that output.

The fix is simple, to use git_color_default_config() as the fallback for
git_add_config(). A more robust change would instead encapsulate the
git_use_color_default global in methods that would check the config
setting if it has not been initialized yet. Some ideas are being
discussed on this front [1], but nothing has been finalized.

[1] https://lore.kernel.org/git/pull.1539.git.1685716420.gitgitgadget@gmail.com/

This test case naturally bisects down to 0527ccb1b55 (add -i: default to
the built-in implementation, 2021-11-30), but the fix makes it clear
that this would be broken even if we added the config to use the builtin
earlier than this.

Reported-by: Greg Alexander <gitgreg@galexander.org>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>