wt-status: load diff ui config

wt-status: load diff ui config

When "git status -v" shows a diff, we did not respect the
user's usual diff preferences at all. Loading just
git_diff_basic_config would give us things like rename
limits and diff drivers. But it makes even more sense to
load git_diff_ui_config, which gives us colorization if the
user has requested it.

Note that we need to take special care to cancel
colorization when writing to the commit template file, as
described in the code comments.

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