Merge branch 'rs/oidset-on-khash'

The oidset API was built on top of the oidmap API which in turn is
on the hashmap API.  Replace the implementation to build on top of
the khash API and gain performance.

* rs/oidset-on-khash:
  oidset: uninline oidset_init()
  oidset: use khash
  khash: factor out kh_release_*
  fetch-pack: load tip_oids eagerly iff needed
  fetch-pack: factor out is_unmatched_ref()