dispatch: make loading extra information from extension extensible

dispatch: make loading extra information from extension extensible

This patch makes loading extra information from extension module at
dispatching extensible. Factoring 'loadcmdtable()' into commands.py is
a part of generalization of loading extra information.

This extensibility assumes registration of new function like below,
for example:

  - revset predicate
  - fileset predicate
  - template keyword
  - template filter
  - template function
  - internal merge tool
  - web command

This patch requires not loader function itself but container module
and the name of it, because listing loader function directly up
implies actual loading module of it, even if it isn't used at runtime
(for example, extensions don't always define revset predicate)

--- a/mercurial/commands.py	Tue Mar 08 00:20:08 2016 -0800
+++ b/mercurial/commands.py	Tue Mar 08 23:04:53 2016 +0900
@@ -7147,3 +7147,12 @@
             for i, name in enumerate(names):
                 ui.write("  %-*s  %s  %s\n" %
                          (maxnamelen, name, place[i], vers[i]))
+
+def loadcmdtable(ui, name, cmdtable):
+    """Load command functions from specified cmdtable
+    """
+    overrides = [cmd for cmd in cmdtable if cmd in table]
+    if overrides:
+        ui.warn(_("extension '%s' overrides commands: %s\n")
+                % (name, " ".join(overrides)))
+    table.update(cmdtable)

--- a/mercurial/dispatch.py	Tue Mar 08 00:20:08 2016 -0800
+++ b/mercurial/dispatch.py	Tue Mar 08 23:04:53 2016 +0900
@@ -743,6 +743,17 @@
                                   [], {})
 
 _loaded = set()
+
+# list of (objname, loadermod, loadername) tuple:
+# - objname is the name of an object in extension module, from which
+#   extra information is loaded
+# - loadermod is the module where loader is placed
+# - loadername is the name of the function, which takes (ui, extensionname,
+#   extraobj) arguments
+extraloaders = [
+    ('cmdtable', commands, 'loadcmdtable'),
+]
+
 def _dispatch(req):
     args = req.args
     ui = req.ui
@@ -772,12 +783,10 @@
     # (uisetup and extsetup are handled in extensions.loadall)
 
     for name, module in exts:
-        cmdtable = getattr(module, 'cmdtable', {})
-        overrides = [cmd for cmd in cmdtable if cmd in commands.table]
-        if overrides:
-            ui.warn(_("extension '%s' overrides commands: %s\n")
-                    % (name, " ".join(overrides)))
-        commands.table.update(cmdtable)
+        for objname, loadermod, loadername in extraloaders:
+            extraobj = getattr(module, objname, None)
+            if extraobj is not None:
+                getattr(loadermod, loadername)(ui, name, extraobj)
         _loaded.add(name)
 
     # (reposetup is handled in hg.repository)