Re: [PATCH 05/10] 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.
>