refs.h: implement `hidden_refs_to_excludes()`

In subsequent commits, we'll teach `receive-pack` and `upload-pack` to
use the new jump list feature in the packed-refs iterator by ignoring
references which are mentioned via its respective hideRefs lists.

However, the packed-ref jump lists cannot handle un-hiding rules (that
begin with '!'), or namespace comparisons (that begin with '^'). Add a
convenience function to the refs.h API to detect when either of these
conditions are met, and returns an appropriate value to pass as excluded
patterns.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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