reftable/merged: skip comparison for records of the same subiter

reftable/merged: skip comparison for records of the same subiter

When retrieving the next entry of a merged iterator we need to drop all
records of other sub-iterators that would be shadowed by the record that
we are about to return. We do this by comparing record keys, dropping
all keys that are smaller or equal to the key of the record we are about
to return.

There is an edge case here where we can skip that comparison: when the
record in the priority queue comes from the same subiterator as the
record we are about to return then we know that its key must be larger
than the key of the record we are about to return. This property is
guaranteed by the sub-iterators, and if it didn't hold then the whole
merged iterator would return records in the wrong order, too.

While this may seem like a very specific edge case it's in fact quite
likely to happen. For most repositories out there you can assume that we
will end up with one large table and several smaller ones on top of it.
Thus, it is very likely that the next entry will sort towards the top of
the priority queue.

Special case this and break out of the loop in that case. The following
benchmark uses git-show-ref(1) to print a single ref matching a pattern
out of 1 million refs:

  Benchmark 1: show-ref: single matching ref (revision = HEAD~)
    Time (mean ± σ):     162.6 ms ±   4.5 ms    [User: 159.0 ms, System: 3.5 ms]
    Range (min … max):   156.6 ms … 188.5 ms    1000 runs

  Benchmark 2: show-ref: single matching ref (revision = HEAD)
    Time (mean ± σ):     156.8 ms ±   4.7 ms    [User: 153.0 ms, System: 3.6 ms]
    Range (min … max):   151.4 ms … 188.4 ms    1000 runs

  Summary
    show-ref: single matching ref (revision = HEAD) ran
      1.04 ± 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>