doc: make links for hg role

The system has a bit of smartness:

  :hg:`update`     -> links to hg.1.html#update
  :hg:`help diffs` -> links to hg.1.html#diffs

doc/runrst

@@ -1,35 +1,42 b''
 #!/usr/bin/env python
 #
 # runrst - register custom roles and run correct writer
 #
 # Copyright 2010 Matt Mackall <[e-mail]> and others
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """usage: %s WRITER args...
 
 where WRITER is the name of a Docutils writer such as 'html' or 'manpage'
 """
 
 import sys
 from docutils.parsers.rst import roles
 from docutils.core import publish_cmdline
 from docutils import nodes, utils
 
 def role_hg(name, rawtext, text, lineno, inliner,
             options={}, content=[]):
-    node = nodes.literal(rawtext, "hg " + utils.unescape(text, True))
+    text = "hg " + utils.unescape(text)
+    linktext = nodes.literal(rawtext, text)
+    parts = text.split()
+    cmd, args = parts[1], parts[2:]
+    if cmd == 'help' and args:
+        cmd = args[0] # link to 'dates' for 'hg help dates'
+    node = nodes.reference(rawtext, '', linktext,
+                           refuri="hg.1.html#%s" % cmd)
     return [node], []
 
 roles.register_local_role("hg", role_hg)
 
 if __name__ == "__main__":
     if len(sys.argv) < 2:
         sys.stderr.write(__doc__ % sys.argv[0])
         sys.exit(1)
 
     writer = sys.argv[1]
     del sys.argv[1]
 
     publish_cmdline(writer_name=writer)