extensions: indicate loaded for an immediately called afterload callback

Otherwise, there's no way to tell between the immediate callback when it is
already loaded, and when the extension is not loaded at all.

mercurial/extensions.py

@@ -148,7 +148,7 b' def afterloaded(extension, callback):'
     '''
 
     if extension in _extensions:
-        callback(loaded=False)
+        callback(loaded=True)
     else:
         _aftercallbacks.setdefault(extension, []).append(callback)