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.)