help: make helptable a list instead of a tuple

The table should always have been a list so that extensions can append
their own help topics -- hgsubversion would like to do this now.

mercurial/help.py

@@ -82,7 +82,7 b' def loaddoc(topic):'
         return gettext(open(path).read())
     return loader
 
-helptable = (
+helptable = [
     (["config"], _("Configuration Files"), loaddoc('config')),
     (["dates"], _("Date Formats"), loaddoc('dates')),
     (["patterns"], _("File Name Patterns"), loaddoc('patterns')),
@@ -100,4 +100,4 b' helptable = ('
     (["extensions"], _("Using additional features"), extshelp),
     (["hgweb"], _("Configuring hgweb"), loaddoc('hgweb')),
     (["glossary"], _("Glossary"), loaddoc('glossary')),
-)
+]