Add initial version of git commit message preparation hook

Start with a slightly modified version of the SVN commit template, to
allow developers to experiment.  This will be updated in the future as
our process and techniques evolve.

This can be installed by copying or symlinking into the .git/hooks/
directory.

Feedback from:	cem, jhb
Reviewed by:	imp
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D27633

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

@@ -0,0 +1,55 @@
+#!/bin/sh
+
+case "$2" in
+commit|message)
+	# It appears git invokes this script for interactive rebase but does
+	# not remove commented lines, so just exit if we're not called with the
+	# default (comment-containing) template.
+	egrep -q '^#' "$1" || return 0
+	;;
+template)
+	return 0
+	;;
+merge)
+	return 0
+	;;
+esac
+
+outfile=$(mktemp /tmp/freebsd-git-commit.XXXXXXXX)
+cat >$outfile <<EOF
+
+$(awk '1;/^#$/{exit}' $1)
+# Uncomment and complete these metadata fields, as appropriate:
+# 
+# PR:		
+# Submitted by:	
+# Reported by:	
+# Reviewed by:	
+# Approved by:	
+# Obtained from:	
+# MFC after:	
+# MFH:		
+# Relnotes:	
+# Security:	
+# Sponsored by:	
+# Pull Request:	
+# Differential Revision:	
+#
+# Description of fields to fill in above:                 72 columns --|
+# PR:                       If and which Problem Report is related.
+# Submitted by:             If someone else sent in the change.
+# Reported by:              If someone else reported the issue.
+# Reviewed by:              If someone else reviewed your modification.
+# Approved by:              If you needed approval for this commit.
+# Obtained from:            If the change is from a third party.
+# MFC after:                N [day[s]|week[s]|month[s]].  Request a reminder email.
+# MFH:                      Ports tree branch name.  Request approval for merge.
+# Relnotes:                 Set to 'yes' for mention in release notes.
+# Security:                 Vulnerability reference (one per line) or description.
+# Sponsored by:             If the change was sponsored by an organization.
+# Pull Request:             https://github.com/freebsd/<repo>/pull/### (*full* GitHub URL needed).
+# Differential Revision:    https://reviews.freebsd.org/D### (*full* phabric URL needed).
+$(awk '/^#$/,EOF' $1)
+EOF
+
+mv $outfile $1