prepare-commit-msg: Sync with committers guide

Add `Discussed with:`, `Closes:`, `MFC to:`, and `Co-authored-by:` to
the commit message template from the committer's guide. While here,
wordsmith these to fit on standard console.

Reported by:		lwhsu
Discussed with:		emaste, jlduran
Reviewed by:		vexeduxr
Differential Revision:	https://reviews.freebsd.org/D54707

tools/tools/git/hooks/prepare-commit-msg

@@ -42,24 +42,29 @@ $(awk '1;/^#$/{exit}' $1)
 #
 # Uncomment and complete these metadata fields, as appropriate:
 #
-# PR:		<If and which Problem Report is related.>
-# Reported by:	<If someone else reported the issue.>
-# Reviewed by:	<If someone else reviewed your modification.>
-# Tested by:	<If someone else tested the change.>
-# Approved by:	<If you needed approval for this commit.>
-# Obtained from:	<If the change is from a third party.>
-# Fixes:		<Short hash and title line of commit fixed by this change>
-# MFC after:	<N [day[s]|week[s]|month[s]].  Request a reminder email>
-# Relnotes:	<Set to 'yes' for mention in release notes.>
-# Security:	<Vulnerability reference (one per line) or description.>
-# Event:		<If this was done at a conference.>
-# Sponsored by:	<If the change was sponsored by an organization.>
+# PR:		<If and which Problem Report is related>
+# Reported by:	<If someone else reported the issue>
+# Reviewed by:	<If someone else reviewed your modification>
+# Tested by:	<If someone else tested the change>
+# Discussed with:	<If someone else provided meaningful feedback>
+# Approved by:	<If you needed approval for this commit>
+# Obtained from:	<If the change is from a third party>
+# Fixes:		<Short hash + commit title fixed by this change>
+# MFC after:	<N [day[s]|week[s]|month[s]] (Request a reminder email)>
+# MFC to:	<Specify specific branches to MFC to>
+# Relnotes:	<Set to 'yes' for mention in release notes>
+# Security:	<Vulnerability reference (one per line) or description>
+# Event:		<If this was done at a conference>
+# Sponsored by:	<If the change was sponsored by an organization>
 # Pull Request:	<https://github.com/freebsd/<repo>/pull/###>
+# Closes:	<https://github.com/freebsd/<repo>/pull/###>
+# Co-authored-by:	<Name and email address of an additional author>
 # Differential Revision:	<https://reviews.freebsd.org/D###>
 #
-# "Pull Request" and "Differential Revision" require the *full* GitHub or
-# Phabricator URL.  The commit author should be set appropriately, using
-# \`git commit --author\` if someone besides the committer sent in the change.
+# "Pull Request", "Closes" and "Differential Revision" require the
+# *full* GitHub or Phabricator URL.  The commit author should be set
+# appropriately, using \`git commit --author\` if someone besides the
+# committer sent in the change.
 $(awk '/^#$/,EOF' "$1")
 EOF