reftable/merged: refactor seeking of records

reftable/merged: refactor seeking of records

The code to seek reftable records in the merged table code is quite hard
to read and does not conform to our coding style in multiple ways:

  - We have multiple exit paths where we release resources even though
    that is not really necessary.

  - We use a scoped error variable `e` which is hard to reason about.
    This variable is not required at all.

  - We allocate memory in the variable declarations, which is easy to
    miss.

Refactor the function so that it becomes more maintainable in the
future.

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