consider improving some TOCTOU subtleties

[ee7]

For example, if a user:

1. Runs configlet sync -u --tests -e foo
2. Waits at the prompt
3. Changes the tests.toml file for exercise foo in their track directory, e.g. adding include = false to a test.
4. Continues going through configlet prompts until completion

If I recall correctly, configlet sync still works, and ignores that the file was changed. We could document this, or consider producing a warning if the file was modified since read-time. But it's probably tricky (and not worth trying) to support arbitrary changes to a file that we're asking questions about.