t/helper: inline `reftable_dump_main()`

t/helper: inline `reftable_dump_main()`

The printing functionality part of `reftable/dump.c` is really only used
by our "dump-reftable" test helper. It is certainly not generic logic
that is useful to anybody outside of Git, and the format it generates is
quite specific. Still, parts of it are used in our test suite and the
output may be useful to take a peek into reftable stacks, tables and
blocks. So while it does not make sense to expose this as part of the
reftable library, it does make sense to keep it around.

Inline the `reftable_dump_main()` function into the "dump-reftable" test
helper. This clarifies that its format is subject to change and not part
of our public interface. Furthermore, this allows us to iterate on the
implementation in subsequent patches.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>