Support configuration updates

[asharov]

Even with the ability to list what files are considered "source" and "tests", it is not always possible to get the configuration file correct on the first try. Currently, if the configuration for already-processed commits needs to be changed, Git Hammer needs to be rerun from scratch. For large repositories, this will waste a lot of time.

It could be possible to support configuration updates for existing data. This would mean processing only files whose status changed between source and non-source, or test and non-test. Especially with small changes in the set of source and test files, this should save a significant amount of time compared to rerunning the whole process.