Add Auto-Submitted header to post-receive-email

This conforms to RFC3834 and is useful in preventing eg
vacation auto-responders from replying by default

Signed-off-by: Chris Hiestand <chiestand@salk.edu>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: chrishiestand

contrib/hooks/post-receive-email

@@ -237,6 +237,7 @@ generate_email_header()
 	X-Git-Reftype: $refname_type
 	X-Git-Oldrev: $oldrev
 	X-Git-Newrev: $newrev
+	Auto-Submitted: auto-generated
 
 	This is an automated email from the git hooks/post-receive script. It was
 	generated because a ref change was pushed to the repository containing