What's new:
* Add support for building on and for Darwin:arm64.
* Change `git-sizer --help` to return success.
* Allow arbitrary references to be included/excluded from the
  analysis.
* Use a safer method to locate the `git` executable.

Internal changes:
* Upgrade to the Go 1.16 toolchain and the use of Go modules.
* Better decouple the test and production code.