Git commit message templates

[Ed Maste]

I've written up an article on commit messages that's now incorporated
into Warner's doc collection, with an intended final destination of
the handbook. The current version of that document is here:
https://github.com/bsdimp/freebsd-git-docs/blob/main/commit.md

One outstanding Git transition item is work on commit message
templates. We currently have an SVN template that lists the metadata
fields we use, such as PR:, Reviewed by:, and such. Git supports local
commit hooks (in .git/hooks) and I've made a trivial port of that to a
git commit message preparation hook. I imagine we'd have these in the
tree (in tools/tools/git/hooks/) and provide either documentation or a
script to install or symlink them into the user's .git/hooks/
directory.

The current trivial hook is open in https://reviews.freebsd.org/D27633.