Support for overwrite operations

[amireh]

This can be achieved either by adding a whole new operation type, or by combining a DELETE op with a CREATE one. In the latter case, whenever a CREATE op is staged and finds out that the target already exists, it has to check the repository for any DELETE ops with the target as its own target.

Clearly, the second option is more complex and could be messy as things will go bad if the order of operations in the script was not properly set.