help: check if a subtopic exists and raise an error if it doesn't (issue6145)

Differential Revision: https://phab.mercurial-scm.org/D6441

mercurial/help.py

@@ -689,6 +689,8 b' def help_(ui, commands, name, unknowncmd'
             for names, header, doc in subtopics[name]:
                 if subtopic in names:
                     break
+            if not any(subtopic in s[0] for s in subtopics[name]):
+                raise error.UnknownCommand(name)
 
         if not header:
             for topic in helptable: