Code clean-up in refs API implementation.
* mh/tidy-ref-update-flags:
refs: update some more docs to use "oid" rather than "sha1"
write_packed_entry(): take `object_id` arguments
refs: rename constant `REF_ISPRUNING` to `REF_IS_PRUNING`
refs: rename constant `REF_NODEREF` to `REF_NO_DEREF`
refs: tidy up and adjust visibility of the `ref_update` flags
ref_transaction_add_update(): remove a check
ref_transaction_update(): die on disallowed flags
prune_ref(): call `ref_transaction_add_update()` directly
files_transaction_prepare(): don't leak flags to packed transaction
a97222978a