built-in add -p: show colored hunks by default

built-in add -p: show colored hunks by default

Just like the Perl version, we now generate two diffs if `color.diff` is
set: one with and one without color. Then we parse them in parallel and
record which hunks start at which offsets in both.

Note that this is a (slight) deviation from the way the Perl version did
it: we are no longer reading the output of `diff-files` line by line
(which is more natural for Perl than for C), but in one go, and parse
everything later, so we might just as well do it in synchrony.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>