log: add an extension hook-point in changeset_printer Extensions sometimes wants to add other information in the default log output format (when no templating is used). Add an empty function named '_exthook' for easing the extension life. Extensions will be able to wrap this function and collaborate to display additional information. Exthook is called after displaying troubles and just before displaying the files, extra and description. Add a new test file to test it and not pollute other test files.