Track origin of copied files

[thiloplanz]

When a file is copied, a new "version 1" is created. There should be some meta data pointing at the source file (and revision) of which the copy was made.

This mechanism could also be for other types of changes where the previous revision is not simply the one with the preceding version number on the same object, for example in "revert" type operations, where the base revision is an earlier one.