I think we can make this work, but it's tricky to debug and a verrrry low priority.

On Windows, the diff binary may not exist. It generally will in a Git Bash environment but not in a cmd.exe environment. At least for now, just fall back to something reasonable-ish here (when printing test differences). Elsewhere we skip a couple tests which need diff.

This version of echo emits its arguments in a way that is unambiguously parseable. I expect to take advantage of this once Windows argument parsing gets test coverage.