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 d6fc6fe
(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>

Author: jltobler

odb/transaction.c

@@ -1,13 +1,13 @@
 #include "git-compat-util.h"
-#include "object-file.h"
+#include "odb/source.h"
 #include "odb/transaction.h"
 
 struct odb_transaction *odb_transaction_begin(struct object_database *odb)
 {
 	if (odb->transaction)
 		return NULL;
 
-	odb->transaction = odb_transaction_files_begin(odb->sources);
+	odb_source_begin_transaction(odb->sources, &odb->transaction);
 
 	return odb->transaction;
 }