deposit: record merge customization
deposit: record merge customization

* Adds option to override default merging of deposition into a record
  after editing.

* Renames variables to improve readability of merge_record task.

Signed-off-by: Lars Holm Nielsen <7eaffca9f7cd98839b5821d0d23e09ff764b82cd@cern.ch>

inveniosoftware/invenio-deposit,inveniosoftware/invenio-deposit,inveniosoftware/invenio-deposit,inveniosoftware/invenio-deposit