commands: Check if helptext contains a newline before we split

mercurial/commands.py

@@ -1555,6 +1555,9 b' def help_(ui, name=None, with_version=Fa'
             raise error.UnknownCommand(name)
 
         doc = gettext(mod.__doc__) or _('no help text available')
+        if '\n' not in doc:
+            head, tail = doc, ""
+        else:
         head, tail = doc.split('\n', 1)
         ui.write(_('%s extension - %s\n\n') % (name.split('.')[-1], head))
         if tail: