refs/reftable: precompute prefix length

refs/reftable: precompute prefix length

We're recomputing the prefix length on every iteration of the ref
iterator. Precompute it for another speedup when iterating over 1
million refs:

    Benchmark 1: show-ref: single matching ref (revision = HEAD~)
      Time (mean ± σ):     100.3 ms ±   3.7 ms    [User: 97.3 ms, System: 2.8 ms]
      Range (min … max):    97.5 ms … 139.7 ms    1000 runs

    Benchmark 2: show-ref: single matching ref (revision = HEAD)
      Time (mean ± σ):      95.8 ms ±   3.4 ms    [User: 92.9 ms, System: 2.8 ms]
      Range (min … max):    93.0 ms … 121.9 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>