Add authors file for informative author fields with git log

From the top of the ports tree run
git config svn.authorsfile .gitauthors
and git log will show author fields that look like
Rodney Grimes <rgrimes@FreeBSD.org>
instead of
user <user@aed309b6-a8cd-e111-996c-001c23d10e55>.

Warning: If a commit is made by an author who is not included in this
file, git svn dcommit will fail until the author has been added.

This file is append-only. Anyone who has ever committed or has commit
access to any of the repositories should be included. The only exception
when users could be removed is if they no longer have an account and
never committed.

TODO: - Add peter's script [1] to a post-commit hook, so that additions to

  svnadmin/conf/access will be reflected in .gitauthors as soon as
  possible.
- Add peter's script to a cron job on freefall to update .gitauthors
  when users change their gecos information.

[1] Latest version of the script can be found at

freefall:/home/jrm/update_gitauthors.pl

Reviewed by: eadler karels mat peter rgrimes swills
Differential Revision: https://reviews.freebsd.org/D13754