hooks/post-receive-email: set declared encoding to utf-8

Some email clients (e.g., claws-mail) display the message body
incorrectly when the charset is not defined explicitly in a
Content-Type header.  "git log" generates logs in UTF-8 encoding by
default, so add a Content-Type header declaring that encoding to
the emails the post-receive-email example hook sends.

[jn: also setting the Content-Transfer-Encoding so MTAs know what
 kind of mangling might be needed when sending to a non 8-bit clean
 SMTP host]

Requested-by: Alexander Gerasiov <gq@debian.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/hooks/post-receive-email

@@ -242,6 +242,9 @@ generate_email_header()
 	cat <<-EOF
 	To: $recipients
 	Subject: ${emailprefix}$projectdesc $refname_type $short_refname ${change_type}d. $describe
+	MIME-Version: 1.0
+	Content-Type: text/plain; charset=utf-8
+	Content-Transfer-Encoding: 8bit
 	X-Git-Refname: $refname
 	X-Git-Reftype: $refname_type
 	X-Git-Oldrev: $oldrev