help: teach loaddoc to load from a different directory

The help system currently only supports showing help topics from a
single directory. We'll need to teach it to show results from
different directories in order to show the internals topics.

The first step is to teach loaddoc() to load documentation from
a sub-directory.

mercurial/help.py

@@ -146,11 +146,13 b' def topicmatch(ui, kw):'
                 results['extensioncommands'].append((cmdname, cmddoc))
     return results
 
-def loaddoc(topic):
+def loaddoc(topic, subdir=None):
     """Return a delayed loader for help/topic.txt."""
 
     def loader(ui):
         docdir = os.path.join(util.datapath, 'help')
+        if subdir:
+            docdir = os.path.join(docdir, subdir)
         path = os.path.join(docdir, topic + ".txt")
         doc = gettext(util.readfile(path))
         for rewriter in helphooks.get(topic, []):