range-diff: offer to dual-color the diffs

range-diff: offer to dual-color the diffs

When showing what changed between old and new commits, we show a diff of
the patches. This diff is a diff between diffs, therefore there are
nested +/- signs, and it can be relatively hard to understand what is
going on.

With the --dual-color option, the preimage and the postimage are colored
like the diffs they are, and the *outer* +/- sign is inverted for
clarity.

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