reftable/merged: avoid duplicate pqueue emptiness check

reftable/merged: avoid duplicate pqueue emptiness check

When calling `merged_iter_next_void()` we first check whether the iter
has been exhausted already. We already perform this check two levels
down the stack in `merged_iter_next_entry()` though, which makes this
check redundant.

Now if this check was there to accelerate the common case it might have
made sense to keep it. But the iterator being exhausted is rather the
uncommon case because you can expect most reftable stacks to contain
more than two refs.

Simplify the code by removing the check. As `merged_iter_next_void()` is
basically empty except for calling `merged_iter_next()` now, merge these
two functions. This also results in a tiny speedup when iterating over
many refs:

    Benchmark 1: show-ref: single matching ref (revision = HEAD~)
      Time (mean ± σ):     125.6 ms ±   3.8 ms    [User: 122.7 ms, System: 2.8 ms]
      Range (min … max):   122.4 ms … 153.4 ms    1000 runs

    Benchmark 2: show-ref: single matching ref (revision = HEAD)
      Time (mean ± σ):     124.0 ms ±   3.9 ms    [User: 121.1 ms, System: 2.8 ms]
      Range (min … max):   120.1 ms … 156.4 ms    1000 runs

    Summary
      show-ref: single matching ref (revision = HEAD) ran
        1.01 ± 0.04 times faster than show-ref: single matching ref (revision = HEAD~)

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