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`