range-diff: right-trim commit messages

range-diff: right-trim commit messages

When comparing commit messages, we need to keep in mind that they are
indented by four spaces. That is, empty lines are no longer empty, but
have "trailing whitespace". When displaying them in color, that results
in those nagging red lines.

Let's just right-trim the lines in the commit message, it's not like
trailing white-space in the commit messages are important enough to care
about in `git range-diff`.

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