help: add web commands to help documentation

The capabilities and URL endpoints of the hgweb server can currently
only be inferred by looking at links in `hg serve` output or by reading
the source code. I've frequently found myself wanting to quickly see
what URLs and capabilities are available.

This patch teaches the help system how to display information about
web commands and their URLs. Using a mechanism similar to revsets,
templates, etc, we can now iterate over the docstrings of registered
web command functions and display them in the help output.

Unfortunately, web commands don't currently have docstrings, so the
output is currently empty. This will be addressed in the following
patches. I apologize for the patch bomb.

mercurial/help.py

@@ -11,6 +11,7 b' import error'
 import extensions, revset, fileset, templatekw, templatefilters, filemerge
 import encoding, util, minirst
 import cmdutil
+import hgweb.webcommands as webcommands
 
 def listexts(header, exts, indent=1, showdeprecated=False):
     '''return a text listing of the given extensions'''
@@ -202,6 +203,7 b" addtopicsymbols('merge-tools', '.. inter"
 addtopicsymbols('revsets', '.. predicatesmarker', revset.symbols)
 addtopicsymbols('templates', '.. keywordsmarker', templatekw.dockeywords)
 addtopicsymbols('templates', '.. filtersmarker', templatefilters.filters)
+addtopicsymbols('hgweb', '.. webcommandsmarker', webcommands.commands)
 
 def help_(ui, name, unknowncmd=False, full=True, **opts):
     '''

mercurial/help/hgweb.txt

@@ -77,3 +77,10 b' changeset to operate on. This is commonl'
 12 digit hexidecimal abbreviation for the full 40 character unique
 revision identifier. However, any value described by
 :hg:`help revisions` typically works.
+
+Commands and URLs
+=================
+
+The following web commands and their URLs are available:
+
+  .. webcommandsmarker