Add a utility function for chaining nested setdefault calls.
Several parts of the diff code built deeply nested dictionaries through

awkwardly formatted setdefault calls. This was hard to make both

PEP-8-compliant and readable, and wasn't the nicest way of going about

it.
This introduces a new utility function that takes variable arguments of

keys, and builds out the nested dictionaries. It then sets the last to

be a list, as needed by the diff code for the property storage.

Unit tests pass.