post-receive-email: remove spurious commas in email subject

The previous form produced subjects like

  [SCM] project.git branch, foo, updated. ...

The new one will produce the lighter

  [SCM] project.git branch foo updated. ...
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/hooks/post-receive-email

@@ -203,7 +203,7 @@ generate_email_header()
 	# Generate header
 	cat <<-EOF
 	To: $recipients
-	Subject: ${emailprefix}$projectdesc $refname_type, $short_refname, ${change_type}d. $describe
+	Subject: ${emailprefix}$projectdesc $refname_type $short_refname ${change_type}d. $describe
 	X-Git-Refname: $refname
 	X-Git-Reftype: $refname_type
 	X-Git-Oldrev: $oldrev