Merge branch 'kn/reflog-migration-fix-followup'

Code clean-up.

* kn/reflog-migration-fix-followup:
  reftable: prevent 'update_index' changes after adding records
  refs: use 'uint64_t' for 'ref_update.index'
  refs: mark `ref_transaction_update_reflog()` as static

diff --cc refs.c
Simple merge

diff --cc refs/reftable-backend.c
Simple merge

diff --cc reftable/reftable-writer.h
Simple merge

diff --cc reftable/stack.c
Simple merge

diff --cc reftable/writer.c
Simple merge