refs: plumb `exclude_patterns` argument throughout

refs: plumb `exclude_patterns` argument throughout

The subsequent patch will want to access an optional `excluded_patterns`
array within `refs/packed-backend.c` that will cull out certain
references matching any of the given patterns on a best-effort basis.

To do so, the refs subsystem needs to be updated to pass this value
across a number of different locations.

Prepare for a future patch by introducing this plumbing now, passing
NULLs at top-level APIs in order to make that patch less noisy and more
easily readable.

Signed-off-by: Taylor Blau <me@ttaylorr.co>
Signed-off-by: Junio C Hamano <gitster@pobox.com>