diff-highlight: refactor to prepare for multi-line hunks

diff-highlight: refactor to prepare for multi-line hunks

The current code structure assumes that we will only look at
a pair of lines at any given time, and that the end result
should always be to output that pair. However, we want to
eventually handle multi-line hunks, which will involve
collating pairs of removed/added lines. Let's refactor the
code to return highlighted pairs instead of printing them.

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