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`