[merge] add gnu changelog format with 'log --format changelog'

[Ben Finney]

Martin Pool <mbp at canonical.com> writes:

> This adds support for GNU changelogs contributed by Andrea Bolognani
> in <https://bugs.launchpad.net/bugs/29582>, plus a smoke test from
> me.

Should the format be instead called ‘gnu-changelog’? This would both
make it clear *which* changelog format is being referred to, and leave
it on an equal footing with other common changelog formats that might
be requested in future.

-- 
 \           “If you're a cowboy and you're dragging a guy behind your |
  `\      horse, I bet it would really make you mad if you looked back |
_o__)                and the guy was reading a magazine.” —Jack Handey |
Ben Finney