diff: move diff.wsErrorHighlight to "basic" config

diff: move diff.wsErrorHighlight to "basic" config

We parse diff.wsErrorHighlight in git_diff_ui_config(), meaning that it
doesn't take effect for plumbing commands, only for porcelains like
git-diff itself. This is mildly annoying as it means scripts like
add--interactive, which produce a user-visible diff with color, don't
respect the option.

We could teach that script to parse the config and pass it along as
--ws-error-highlight to the diff plumbing. But there's a simpler
solution.

It should be reasonably safe for plumbing to respect this option, as it
only kicks in when color is otherwise enabled. And anybody parsing
colorized output must already deal with the fact that color.diff.* may
change the exact output they see; those options have been part of
git_diff_basic_config() since its inception in 9a1805a872 (add a "basic"
diff config callback, 2008-01-04).

So we can just move it to the "basic" config, which fixes
add--interactive, along with any other script in the same boat, with a
very low risk of hurting any plumbing users.

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