Switch CLI API dispatch framework to `docopt_dispatch`

The current [command dispatch code](https://github.com/guyzmo/git-repo/blob/devel/git_repo/kwargparse.py) has been designed to avoid an over-complex main function. But it was after coding it that I discovered that the author of docopt implemented a similar framework he called [`docopt_dispatch`](https://github.com/keleshev/docopt-dispatch).

That framework might need some adjustments for custom features built in the current command dispatch code, but it's worth using an existing and common framework than reinventing the wheel.

Start with git_repo/kwargparse.py and compare its dispatch behavior with the docopt_dispatch framework. Identify which custom features must be preserved before deciding whether the framework can replace the current implementation. Done means the CLI dispatch uses the selected framework without losing those custom features and the existing commands continue to work.

Written by the indexing model from the issue text.