reftable/reader: unify indexed and linear seeking

reftable/reader: unify indexed and linear seeking

In `reader_seek_internal()` we either end up doing an indexed seek when
there is one or a linear seek otherwise. These two code paths are
disjunct without a good reason, where the indexed seek will cause us to
exit early.

Refactor the two code paths such that it becomes possible to share a bit
more code between them.

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