Merge branch 'kn/reftable-writer-log-write-verify'

Reftable backend adds check for upper limit of log's update_index.

* kn/reftable-writer-log-write-verify:
  reftable/writer: ensure valid range for log's update_index

diff --cc reftable/writer.c
Simple merge

diff --cc t/unit-tests/t-reftable-readwrite.c
Simple merge

diff --cc t/unit-tests/t-reftable-stack.c
Simple merge