refs: skip collision checks in initial transactions

Reference transactions use `refs_verify_refname_available()` to check
for colliding references. This check consists of two parts:

  - Checks for whether multiple ref updates in the same transaction
    conflict with each other.

  - Checks for whether existing refs conflict with any refs part of the
    transaction.

While we generally cannot avoid the first check, the second check is
superfluous in cases where the transaction is an initial one in an
otherwise empty ref store. The check results in multiple ref reads as
well as the creation of a ref iterator for every ref we're checking,
which adds up quite fast when performing the check for many refs.

Introduce a new flag that allows us to skip this check and wire it up in
such that the backends pass it when running an initial transaction. This
leads to significant speedups when migrating ref storage backends. From
"files" to "reftable":

    Benchmark 1: migrate files:reftable (refcount = 100000, revision = HEAD~)
      Time (mean ± σ):     472.4 ms ±   6.7 ms    [User: 175.9 ms, System: 285.2 ms]
      Range (min … max):   463.5 ms … 483.2 ms    10 runs

    Benchmark 2: migrate files:reftable (refcount = 100000, revision = HEAD)
      Time (mean ± σ):      86.1 ms ±   1.9 ms    [User: 67.9 ms, System: 16.0 ms]
      Range (min … max):    82.9 ms …  90.9 ms    29 runs

    Summary
      migrate files:reftable (refcount = 100000, revision = HEAD) ran
        5.48 ± 0.15 times faster than migrate files:reftable (refcount = 100000, revision = HEAD~)

And from "reftable" to "files":

    Benchmark 1: migrate reftable:files (refcount = 100000, revision = HEAD~)
      Time (mean ± σ):     452.7 ms ±   3.4 ms    [User: 209.9 ms, System: 235.4 ms]
      Range (min … max):   445.9 ms … 457.5 ms    10 runs

    Benchmark 2: migrate reftable:files (refcount = 100000, revision = HEAD)
      Time (mean ± σ):      95.2 ms ±   2.2 ms    [User: 73.6 ms, System: 20.6 ms]
      Range (min … max):    91.7 ms … 100.8 ms    28 runs

    Summary
      migrate reftable:files (refcount = 100000, revision = HEAD) ran
        4.76 ± 0.11 times faster than migrate reftable:files (refcount = 100000, revision = HEAD~)

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

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