submissionDocs: if a submissionDocs already exists in objects, merge instead of nesting

When submissionDocumentation was being moved into the AIP before characterization, Archivematica was using mv. This meant that if the SIP already had a directory by that name, submissionDocumentation would end up nested inside the existing directory instead of merged with it.

This adds a new moveOrMerge clientscript that acts like mv if the destination doesn't exist, and merges the contents if it does.

The first commit here was from I was mistaken about the source of this problem. It's not actually *wrong*, but probably unnecessary; I'll probably just remove it.