Merge branch 'ps/ref-backend-migration-optim'

Merge branch 'ps/ref-backend-migration-optim'

The migration procedure between two ref backends has been optimized.

* ps/ref-backend-migration-optim:
  reftable: rename scratch buffer
  refs: adapt `initial_transaction` flag to be unsigned
  reftable/block: optimize allocations by using scratch buffer
  reftable/block: rename `block_writer::buf` variable
  reftable/writer: optimize allocations by using a scratch buffer
  refs: don't normalize log messages with `REF_SKIP_CREATE_REFLOG`
  refs: skip collision checks in initial transactions
  refs: use "initial" transaction semantics to migrate refs
  refs/files: support symbolic and root refs in initial transaction
  refs: introduce "initial" transaction flag
  refs/files: move logic to commit initial transaction
  refs: allow passing flags when setting up a transaction