phlgitx_refcache: expose hash_ref_pairs

Expose the underlying list of (hash, ref) so that we can manipulate it
directly.

Expose it via accessors so that this uncommon use is easily searched for
with grep etc.

Test Plan:
$ ./precommit.sh
fixup! Arcyd: refactor, inject refcache_repo to abdt_git
fixup! phlgitx_refcache: expose hash_ref_pairs
abdi_processrepoarglist: persist ref cache

Save the ref cache and restore it in the main process when updating is
done.

Test Plan:
$ ./precommit.sh
TEMP: log every git call
Revert "TEMP: Arcyd disable local ref cache"

This reverts commit 304920b98323a8881aaf74ec869b6dba82966beb.