rev-list: add --authorship-order alternative ordering

--date-order is an excellent alternative to --topo-order if you want a feel for
the *actual history*, chronologically, of your project. I use it often, with
--graph as well; it's a great way to get an overview of a project's recent
development history.

However, in a project that rebases various in-development topic-branches often,
it gets hard to demonstrate a *chronological history* of changes to the
codebase, as this always “resets” the COMMITTER_DATE (which --date-order uses)
to the time the rebase happened; which often means ‘last time all of the
topic-branches were rebased on the latest fixes in master.’

Thus, I've added an --authorship-order version of --date-order, which relies
upon the AUTHOR_DATE instead of the COMMITTER_DATE; this means that old commits
will continue to show up chronologically in-order despite rebasing.

> (Note: I'm completely aware that this is a mirror-only repository. I'm using this pureq as an addendum to my mailing-list post.)