Merge branch 'ps/reftable-multi-level-indices-fix'

Write multi-level indices for reftable has been corrected. * ps/reftable-multi-level-indices-fix: reftable: document reading and writing indices reftable/writer: fix writing multi-level indices reftable/writer: simplify writing index records reftable/writer: use correct type to iterate through index entries reftable/reader: be more careful about errors in indexed seeks