This matches the heuristic used by "notmuch config set" to decide if
something is a list.

This change fixes the bug reported at [1].

[1]: id:6O3LTUhoXlrnkPWCtPJCP4cagU7mFVEdyTpcC_37BoSzStlARXDBa7oczy6hB0jyjGjBQvgj_jFV58cw0aNx-jUg1h1O-FQ7820k68C0X4M=@protonmail.com