refs/reftable: skip duplicate name checks

refs/reftable: skip duplicate name checks

All the callback functions which write refs in the reftable backend
perform D/F conflict checks via `refs_verify_refname_available()`. But
in reality we perform these D/F conflict checks a second time in the
reftable library via `stack_check_addition()`.

Interestingly, the code in the reftable library is inferior compared to
the generic function:

  - It is slower than `refs_verify_refname_available()`, even though
    this can probably be optimized.

  - It does not provide a proper error message to the caller, and thus
    all the user would see is a generic "file/directory conflict"
    message.

Disable the D/F conflict checks in the reftable library by setting the
`skip_name_check` write option. This results in a non-negligible speedup
when writing many refs. The following benchmark writes 100k refs in a
single transaction:

  Benchmark 1: update-ref: create many refs (HEAD~)
    Time (mean ± σ):      3.241 s ±  0.040 s    [User: 1.854 s, System: 1.381 s]
    Range (min … max):    3.185 s …  3.454 s    100 runs

  Benchmark 2: update-ref: create many refs (HEAD)
    Time (mean ± σ):      2.878 s ±  0.024 s    [User: 1.506 s, System: 1.367 s]
    Range (min … max):    2.838 s …  2.960 s    100 runs

  Summary
    update-ref: create many refs (HEAD~) ran
      1.13 ± 0.02 times faster than update-ref: create many refs (HEAD)

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