Merge branch 'jt/odb-transaction-per-source'

Transaction to create objects (or not) is currently tied to the repository, but in the future a repository can have multiple object sources, which may have different transaction mechanisms. Make the odb transaction API per object source. * jt/odb-transaction-per-source: odb: transparently handle common transaction behavior odb: prepare `struct odb_transaction` to become generic object-file: rename transaction functions odb: store ODB source in `struct odb_transaction`