odb/transaction: use pluggable `begin_transaction()`

odb/transaction: use pluggable `begin_transaction()`

Each ODB source is expected to provide an ODB transaction implementation
that should be used when starting a transaction. With d6fc6fe6f8
(odb/source: make `begin_transaction()` function pluggable, 2026-03-05),
the `struct odb_source` now provides a pluggable callback for beginning
transactions. Use the callback provided by the ODB source accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>