Implement filter-branch as a method with callback

[alexandernst]

It would be nice to be able to filter-branch inside python instead of having to create bash scripts at runtime and then pass them to get lowlevel git wrapper.

My use case: I want to be able to rewrite authored dates of commits, which is done with filter-branch, but I don't want to generate bash scripts (aka, I want to keep everything python-only).

PS: If there is a way I can rewrite authored dates using only git-python I'd like to hear it.