[MERGE] Add a new hook Commands['extend_command'] for plugins that want to alter commands without overriding the entire command.

[Bundle Buggy]

Bundle Buggy has detected this merge request.

For details, see: 
http://bundlebuggy.aaronbentley.com/project/bzr/request/%3C1234384804.12070.622.camel%40lifeless-64%3E
Project: Bazaar