Code clean-up to remove hardcoded SHA-1 hash from many places.

* jk/oidhash:
  hashmap: convert sha1hash() to oidhash()
  hash.h: move object_id definition from cache.h
  khash: rename oid helper functions
  khash: drop sha1-specific map types
  pack-bitmap: convert khash_sha1 maps into kh_oid_map
  delta-islands: convert island_marks khash to use oids
  khash: rename kh_oid_t to kh_oid_set
  khash: drop broken oid_map typedef
  object: convert create_object() to use object_id
  object: convert internal hash_obj() to object_id
  object: convert lookup_object() to use object_id
  object: convert lookup_unknown_object() to use object_id
  pack-objects: convert locate_object_entry_hash() to object_id
  pack-objects: convert packlist_find() to use object_id
  pack-bitmap-write: convert some helpers to use object_id
  upload-pack: rename a "sha1" variable to "oid"
  describe: fix accidental oid/hash type-punning