range-diff: use color for the commit pairs

range-diff: use color for the commit pairs

Arguably the most important part of `git range-diff`'s output is the
list of commits in the two branches, together with their relationships.

For that reason, tbdiff introduced color-coding that is pretty
intuitive, especially for unchanged patches (all dim yellow, like the
first line in `git show`'s output) vs modified patches (old commit is
red, new commit is green). Let's imitate that color scheme.

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