More advanced splice tool (and command-line configurable renamer)

I've been working at joining two git repositories, with multiple branches, together.
I've updated the `splice_repos.py` script so that it now supports merging branch histories by choosing the latest commit each repository (as long as the repository branches are in sync with each other).
I've also changed `renamer.py` into a script that allows command-line arguments to rename or exclude files, directories and branches
Because `splice_repos` runs in two passes (for efficiency and to aid debugging), I've added an option to reset the id offset in `git_fast_filter.py`, and also allow passing in a file to `fast_import_input`.

I'm not sure if these changes are something you'd want or be interested in; if not I'm happy to make a separate project for them