unpack-objects: use the bulk-checkin infrastructure

The unpack-objects functionality is used by fetch, push, and fast-import
to turn the transfered data into object database entries when there are
fewer objects than the 'unpacklimit' setting.

By enabling an odb-transaction when unpacking objects, we can take advantage
of batched fsyncs.

Here are some performance numbers to justify batch mode for
unpack-objects, collected on a WSL2 Ubuntu VM.

Fsync Mode | Time for 90 objects (ms)
-------------------------------------
       Off | 170
  On,fsync | 760
  On,batch | 230

Note that the default unpackLimit is 100 objects, so there's a 3x
benefit in the worst case. The non-batch mode fsync scales linearly
with the number of objects, so there are significant benefits even with
smaller numbers of objects.

Signed-off-by: Neeraj Singh <neerajsi@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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