git-annex tries to run in a constant amount of memory, however `knownUrls`
loads all urls ever seen into a list, so the more urls there are, the more
memory `git annex importfeed` will need.

This is probably not a big problem in practice, but seems worth doing
something about if somehow possible.

Unfortunately, can't use a bloom filter, because a false positive would
prevent importing an url that has not been imported before. A sqlite
database would work, but would need to be updated whenever the git-annex
branch is changed. --[[Joey]]
