Merge branch 'ps/reftable-write-options'

Merge branch 'ps/reftable-write-options'

The knobs to tweak how reftable files are written have been made
available as configuration variables.

* ps/reftable-write-options:
  refs/reftable: allow configuring geometric factor
  reftable: make the compaction factor configurable
  refs/reftable: allow disabling writing the object index
  refs/reftable: allow configuring restart interval
  reftable: use `uint16_t` to track restart interval
  refs/reftable: allow configuring block size
  reftable/dump: support dumping a table's block structure
  reftable/writer: improve error when passed an invalid block size
  reftable/writer: drop static variable used to initialize strbuf
  reftable: pass opts as constant pointer
  reftable: consistently refer to `reftable_write_options` as `opts`