Merge branch 'rs/object-id'

Merge branch 'rs/object-id'

"uchar [40]" to "struct object_id" conversion continues.

* rs/object-id:
  checkout: convert post_checkout_hook() to struct object_id
  use oidcpy() for copying hashes between instances of struct object_id
  use oid_to_hex_r() for converting struct object_id hashes to hex strings