Adopting "Fixes" trailer

[Ed Maste]

In review D28693 I've proposed adding a "Fixes" trailer to the commit
message template. It is used to identify a commit that is a follow-up bug
fix to a previous commit, and could generally be used instead of "MFC with"
or "X-MFC-with". One nice aspect is that it conveys fix metadata also for
commits that aren't intended for MFC, which may be useful for downstream
projects.

Fixes is already used by a number of other projects. I have proposed adding
to the template
# Fixes: <Short hash and title line of commit fixed by this change>

Please follow up here or in the review with any comments. I'll commit in a
few days if there are no objections.

https://reviews.freebsd.org/D28693