Merge branch 'jt/odb-transaction'

Merge branch 'jt/odb-transaction'

The work to build on the bulk-checkin infrastructure to create many
objects at once in a transaction and to abstract it into the
generic object layer continues.

* jt/odb-transaction:
  odb: add transaction interface
  object-file: update naming from bulk-checkin
  object-file: relocate ODB transaction code
  bulk-checkin: drop flush_odb_transaction()
  builtin/update-index: end ODB transaction when --verbose is specified
  bulk-checkin: remove ODB transaction nesting