refs: use "initial" transaction semantics to migrate refs

Until now, we couldn't use "initial" transaction semantics to migrate
refs because the "files" backend only supported writing regular refs via
the initial transaction because it simply mapped the transaction to a
"packed-refs" transaction. But with the preceding commit, the "files"
backend has learned to also write symbolic and root refs in the initial
transaction by creating a second transaction for all refs that need to
be written as loose refs.

Adapt the code to migrate refs to commit the transaction as an initial
transaction. This results in a signiticant speedup when migrating many
refs:

    Benchmark 1: migrate reftable:files (refcount = 100000, revision = HEAD~)
      Time (mean ± σ):      3.247 s ±  0.034 s    [User: 0.485 s, System: 2.722 s]
      Range (min … max):    3.216 s …  3.309 s    10 runs

    Benchmark 2: migrate reftable:files (refcount = 100000, revision = HEAD)
      Time (mean ± σ):     453.6 ms ±   1.9 ms    [User: 214.6 ms, System: 230.5 ms]
      Range (min … max):   451.5 ms … 456.4 ms    10 runs

    Summary
      migrate reftable:files (refcount = 100000, revision = HEAD) ran
        7.16 ± 0.08 times faster than migrate reftable:files (refcount = 100000, revision = HEAD~)

As the reftable backend doesn't (yet) special-case initial transactions
there is no comparable speedup for that backend.

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

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