odb: add transaction interface

Transactions are managed via the {begin,end}_odb_transaction() function
in the object-file subsystem and its implementation is specific to the
files object source. Introduce odb_transaction_{begin,commit}() in the
odb subsystem to provide an eventual object source agnostic means to
manage transactions.

Update call sites to instead manage transactions through the odb
subsystem. Also rename {begin,end}_odb_transaction() functions to
object_file_transaction_{begin,commit}() to clarify the object source it
supports.

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

odb.h

@@ -185,6 +185,19 @@ struct object_database {
 struct object_database *odb_new(struct repository *repo);
 void odb_clear(struct object_database *o);
 
+/*
+ * Starts an ODB transaction. Subsequent objects are written to the transaction
+ * and not committed until odb_transaction_commit() is invoked on the
+ * transaction. If the ODB already has a pending transaction, NULL is returned.
+ */
+struct odb_transaction *odb_transaction_begin(struct object_database *odb);
+
+/*
+ * Commits an ODB transaction making the written objects visible. If the
+ * specified transaction is NULL, the function is a no-op.
+ */
+void odb_transaction_commit(struct odb_transaction *transaction);
+
 /*
  * Find source by its object directory path. Dies in case the source couldn't
  * be found.