I couldn't get the diff as shown in the git-show recipe. Therefore
update it to what I think it should be. Maybe there is a better way.

Also add a section on how to assemble a git show-like message. It took
me quite some searching in the Python docs to find out how to do it,
especially the date and time part. So this might save people time. I
wanted to add something that gives me a git show --stat equivalent, but
couldn't figure it out.

Make the diff generation more idiomatic and fix the assembling of the
timestamp. git-show normally prints the author time, so use this instead
of the commit time. Also fix how tzinfo is obtained. Of course we have
to use the author's time zone and not some fixed one as I had written
before.

As @jdavid pointed out, Python 3 already provides a tzinfo subclass for
fixed UTC offsets. Incorporate this in the recipe. Leave the old code
with the self-made class, since many people are working with Python 2
and it is harder to find out there.