Adopting "Fixes" trailer

[Ed Maste]

On Thu, 18 Feb 2021 at 11:53, John Baldwin <jhb at freebsd.org> wrote:
>
> > https://reviews.freebsd.org/D28693
>
> Huh, at first I thought this was to auto-close issues on GitHub.  I think
> this is a good idea.  If we had a wrapper script for MFC's it could
> auto-squash those as an option perhaps.

Ah, I think that's typically "Closes: <identifier>"

Indeed we could search for Fixes in an MFC wrapper; the MFC reminder
mails and https://gitmfc.kernelnomicon.org could also be extended to
list related Fixes commits.