reftable/merged: split up initialization and seeking of records

reftable/merged: split up initialization and seeking of records

To initialize a `struct merged_iter`, we need to seek all subiterators
to the wanted record and then add their results to the priority queue
used to sort the records. This logic is split up across two functions,
`merged_table_seek_record()` and `merged_iter_init()`. The scope of
these functions is somewhat weird though, where `merged_iter_init()` is
only responsible for adding the records of the subiterators to the
priority queue.

Clarify the scope of those functions such that `merged_iter_init()` is
only responsible for initializing the iterator's structure. Performing
the subiterator seeks are now part of `merged_table_seek_record()`.

This step is required to move seeking of records into the generic
`struct reftable_iterator` infrastructure.

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