refs/reftable: don't recompute committer ident

refs/reftable: don't recompute committer ident

In order to write reflog entries we need to compute the committer's
identity as it gets encoded in the log record itself. The reftable
backend does this via `git_committer_info()` and `split_ident_line()` in
`fill_reftable_log_record()`, which use the Git config as well as
environment variables to figure out the identity.

While most callers would only call `fill_reftable_log_record()` once or
twice, `write_transaction_table()` will call it as many times as there
are queued ref updates. This can be quite a waste of effort when writing
many refs with reflog entries in a single transaction.

Refactor the code to pre-compute the committer information. This results
in a small speedup when writing 100000 refs in a single transaction:

  Benchmark 1: update-ref: create many refs (HEAD~)
    Time (mean ± σ):      2.895 s ±  0.020 s    [User: 1.516 s, System: 1.374 s]
    Range (min … max):    2.868 s …  2.983 s    100 runs

  Benchmark 2: update-ref: create many refs (HEAD)
    Time (mean ± σ):      2.845 s ±  0.017 s    [User: 1.461 s, System: 1.379 s]
    Range (min … max):    2.803 s …  2.913 s    100 runs

  Summary
    update-ref: create many refs (HEAD) ran
      1.02 ± 0.01 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>