reftable: allow inlining of a few functions

reftable: allow inlining of a few functions

We have a few functions which are basically just accessors to
structures. As those functions are executed inside the hot loop when
iterating through many refs, the fact that they cannot be inlined is
costing us some performance.

Move the function definitions into their respective headers so that they
can be inlined. This results in a performance improvement when iterating
over 1 million refs:

    Benchmark 1: show-ref: single matching ref (revision = HEAD~)
      Time (mean ± σ):     105.9 ms ±   3.6 ms    [User: 103.0 ms, System: 2.8 ms]
      Range (min … max):   103.1 ms … 133.4 ms    1000 runs

    Benchmark 2: show-ref: single matching ref (revision = HEAD)
      Time (mean ± σ):     100.7 ms ±   3.4 ms    [User: 97.8 ms, System: 2.8 ms]
      Range (min … max):    97.8 ms … 124.0 ms    1000 runs

    Summary
      show-ref: single matching ref (revision = HEAD) ran
        1.05 ± 0.05 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>