t5801 (remote-helpers): add test to fetch tags

This used to work, but commit e198b3a740 broke it.

e198b3a740 (fetch: replace string-list used as a look-up table with a hashmap)

Probably all remote helpers that use the import method are affected, but
we didn't catch the issue.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>