Merge branch 'jk/use-oid-pos'

Merge branch 'jk/use-oid-pos'

Code clean-up to ensure our use of hashtables using object names as
keys use the "struct object_id" objects, not the raw hash values.

* jk/use-oid-pos:
  oid_pos(): access table through const pointers
  hash_pos(): convert to oid_pos()
  rerere: use strmap to store rerere directories
  rerere: tighten rr-cache dirname check
  rerere: check dirname format while iterating rr_cache directory
  commit_graft_pos(): take an oid instead of a bare hash