fix hg help <ext> for extension that do not define any command

solve issue462

mercurial/commands.py

@@ -1345,13 +1345,20 b' def help_(ui, name=None, with_version=Fa'
             ui.write(d, '\n')
 
         ui.status('\n')
+
+        try:
+            ct = mod.cmdtable
+        except AttributeError:
+            ui.status(_('no commands defined\n'))
+            return
+
         if ui.verbose:
             ui.status(_('list of commands:\n\n'))
         else:
             ui.status(_('list of commands (use "hg help -v %s" '
                         'to show aliases and global options):\n\n') % name)
 
-        modcmds = dict.fromkeys([c.split('|', 1)[0] for c in mod.cmdtable])
+        modcmds = dict.fromkeys([c.split('|', 1)[0] for c in ct])
         helplist(modcmds.has_key)
 
     if name and name != 'shortlist':

tests/test-notify

@@ -22,6 +22,7 b' baseurl = http://test/'
 foo@bar = *
 EOF
 
+hg help notify
 hg init a
 echo a > a/a
 echo % commit

tests/test-notify.out

@@ -1,3 +1,6 b''
+notify extension - No help text available
+
+no commands defined
 % commit
 adding a
 % clone