'git receive-pack' has been refactored to use ODB transaction
interfaces instead of directly managing 'tmp_objdir' for staging
incoming objects, bringing it closer to being ODB backend agnostic.
* jt/receive-pack-use-odb-transactions:
builtin/receive-pack: stage incoming objects via ODB transactions
builtin/receive-pack: drop redundant tmpdir env
odb/transaction: introduce ODB transaction flags
odb/transaction: add transaction env interface
odb/transaction: propagate commit errors
odb/transaction: propagate begin errors
object-file: propagate files transaction errors
object-file: drop check for inflight transactions
object-file: embed transaction flush logic in commit function
object-file: rename files transaction fsync function
object-file: rename files transaction prepare function
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