Teach git-commit about commit message templates.

These are useful in organizations that enforce particular formats
for commit messages, e.g., to specify bug IDs or test plans.
Use of the template is not enforced; it is simply used as the
initial content when the editor is invoked.

Signed-off-by: Steven Grimm <koreth@midwinter.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7500/add-signed-off

@@ -0,0 +1,3 @@
+#!/bin/sh
+echo "Signed-off-by: foo <bar@frotz>" >> "$1"
+exit 0