dispatch: allow extensions to provide setup code

we'll need this soon, when record extension will optionally depend
on mq early -- when preparing cmdtable.

Also, if accepted, ExtensionHowto wiki should be updated as well.

mercurial/dispatch.py

@@ -275,6 +275,15 b' def _dispatch(ui, args):'
     for name, module in extensions.extensions():
         if name in _loaded:
             continue
+
+        # setup extensions
+        # TODO this should be generalized to scheme, where extensions can
+        #      redepend on other extensions.  then we should toposort them, and
+        #      do initialization in correct order
+        extsetup = getattr(module, 'extsetup', None)
+        if extsetup:
+            extsetup()
+
         cmdtable = getattr(module, 'cmdtable', {})
         overrides = [cmd for cmd in cmdtable if cmd in commands.table]
         if overrides: