Merge branch 'mh/iterate-refs'

Merge branch 'mh/iterate-refs'

* mh/iterate-refs:
  refs.c: make create_cached_refs() static
  Retain caches of submodule refs
  Store the submodule name in struct cached_refs
  Allocate cached_refs objects dynamically
  Change the signature of read_packed_refs()
  Access reference caches only through new function get_cached_refs()
  Extract a function clear_cached_refs()