Experiment: Convert .gitignore entries to radon ignore/exlude

[devdanzin]

This PR addresses a TODO in build.py: "Convert .gitignore to radon ignore patterns to make the build more efficient". However, timings show no difference in performance even with a lot of added Python files in ignored directories. That happens because radon is called only with modified files from given revisions as targets, so the build is already optimal in this sense.

This PR serves only to document this exploration. Leaving it open for a while in case we can identify a scenario it would help with.