Details here http://superuser.com/questions/1064701/how-to-specify-exact-
matching-in-gnupg-2-1s-quick-sign-key#1064701
TL;DR: matching is always fuzzy. Specifying name signs all keys that contain
name as a substring. = doesn't work to restrict to exact matching. uid
hashes (as listed with -k --with-colons) don't work (in specifing an exact
uid to --quick-sign-key) at all.