help: pass subtopic into help()

Now that we have multiple directories where help topics can live,
we need a mechanism to access them. We already use "." to
separate topic from section. So it seems logical to also use "." to
denote the sub-directory of a topic.

This patch teaches the help command to parse out the possible
sub-topic and pass it to the help system.

mercurial/commands.py

@@ -4367,11 +4367,16 b' def help_(ui, name=None, **opts):'
         keep.append(sys.platform.lower())
 
     section = None
+    subtopic = None
     if name and '.' in name:
         name, section = name.split('.', 1)
         section = section.lower()
-
-    text = help.help_(ui, name, **opts)
+        if '.' in section:
+            subtopic, section = section.split('.', 1)
+        else:
+            subtopic = section
+
+    text = help.help_(ui, name, subtopic=subtopic, **opts)
 
     formatted, pruned = minirst.format(text, textwidth, keep=keep,
                                        section=section)

mercurial/help.py

@@ -255,7 +255,7 b" addtopicsymbols('templates', '.. functio"
 addtopicsymbols('hgweb', '.. webcommandsmarker', webcommands.commands,
                 dedent=True)
 
-def help_(ui, name, unknowncmd=False, full=True, **opts):
+def help_(ui, name, unknowncmd=False, full=True, subtopic=None, **opts):
     '''
     Generate the help for 'name' as unformatted restructured text. If
     'name' is None, describe the commands available.