docs/rev-list: add an examples section

docs/rev-list: add an examples section

We currently don't show any examples of using git-rev-list at all. Let's
add some pretty elementary examples. They likely seem obvious to anybody
who has worked with the tool for a while, but my purpose here is
two-fold:

  - they may be enlightening to people who haven't used the tool a lot
    to give a general flavor of how it is meant to be used

  - they can serve as a starting point for adding more interesting
    examples (we can do that without the basic ones, of course, but I
    think it makes sense to show off the building blocks)

This set is far from exhaustive, but again, the purpose is to be a
starting point for further additions.

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