fetch: fix regression with transport helpers

fetch: fix regression with transport helpers

Commit e198b3a740 changed the behavior of fetch with regards to tags.
Before, null oids where not ignored, now they are, regardless of whether
the refs have been explicitly cleared or not.

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

When using a transport helper the oids can certainly be null. So now
tags are ignored and fetching them is impossible.

This patch fixes that by having a specific flag that is set only when we
explicitly want to ignore the refs, restoring the original behavior.

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