object-file: embed transaction flush logic in commit function

object-file: embed transaction flush logic in commit function

When a "files" transaction is committed,
`flush_loose_object_transaction()` is invoked to handle performing a
hardware flush along with migrating the temporary object directory into
the primary and configuring the repository ODB source accordingly. The
function name here is a bit misleading because the helper is doing a bit
more than just "flushing" the transaction contents. Also, in a
subsequent commit, the transaction temporary directory is used to stage
packfiles and not just loose objects anymore.

Lift the helper function logic into `odb_transaction_files_commit()` to
more accurately signal to readers the operation being performed.

Signed-off-by: Justin Tobler <redacted>
Signed-off-by: Junio C Hamano <redacted>