reftable/writer: optimize allocations by using a scratch buffer

reftable/writer: optimize allocations by using a scratch buffer

Both `writer_add_record()` and `reftable_writer_add_ref()` get executed
for every single ref record we're adding to the reftable writer. And as
both functions use a local buffer to write data, the allocations we have
to do here add up during larger transactions.

Refactor the code to use a scratch buffer part of the `reftable_writer`
itself such that we can reuse it. This signifcantly reduces the number
of allocations during large transactions, e.g. when migrating refs from
the "files" backend to the "reftable" backend. Before this change:

    HEAP SUMMARY:
        in use at exit: 80,048 bytes in 49 blocks
      total heap usage: 5,032,171 allocs, 5,032,122 frees, 418,792,092 bytes allocated

After this change:

    HEAP SUMMARY:
        in use at exit: 80,048 bytes in 49 blocks
      total heap usage: 3,025,864 allocs, 3,025,815 frees, 372,746,291 bytes allocated

This also translate into a small speedup:

    Benchmark 1: migrate files:reftable (refcount = 1000000, revision = HEAD~)
      Time (mean ± σ):     827.2 ms ±  16.5 ms    [User: 689.4 ms, System: 124.9 ms]
      Range (min … max):   809.0 ms … 924.7 ms    50 runs

    Benchmark 2: migrate files:reftable (refcount = 1000000, revision = HEAD)
      Time (mean ± σ):     813.6 ms ±  11.6 ms    [User: 679.0 ms, System: 123.4 ms]
      Range (min … max):   786.7 ms … 833.5 ms    50 runs

    Summary
      migrate files:reftable (refcount = 1000000, revision = HEAD) ran
        1.02 ± 0.02 times faster than migrate files:reftable (refcount = 1000000, revision = HEAD~)

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