stgit.el: Make "C-u r" raise error if run with prefix argument without a patch

[stgit] / templates / patchmail.tmpl

From: %(sender)s
Subject: [%(prefix)sPATCH%(version)s%(number)s] %(shortdescr)s

%(fromauth)s%(longdescr)s
---
%(diffstat)s
%(diff)s