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>