Merge branch 'kn/non-transactional-batch-updates'

Merge branch 'kn/non-transactional-batch-updates'

Updating multiple references have only been possible in all-or-none
fashion with transactions, but it can be more efficient to batch
multiple updates even when some of them are allowed to fail in a
best-effort manner.  A new "best effort batches of updates" mode
has been introduced.

* kn/non-transactional-batch-updates:
  update-ref: add --batch-updates flag for stdin mode
  refs: support rejection in batch updates during F/D checks
  refs: implement batch reference update support
  refs: introduce enum-based transaction error types
  refs/reftable: extract code from the transaction preparation
  refs/files: remove duplicate duplicates check
  refs: move duplicate refname update check to generic layer
  refs/files: remove redundant check in split_symref_update()