Provide maximally useful log output.

Per-file log output appears confusing or suboptimal at the moment. The correct output needs to be defined. This needs to consider things like how to render merged revisions that changed the file - whether their visual indent should be preserved, should revisions that merged them be included to allow the depth to step by 1 only ? etc.

Log also fails for non mainline revisions.

And the interface is incomplete, in that some types of formatters (xml) can not be generated with the current api.