Avoid m2m selects when diffing

[sebashwa]

Problem Statement
I'm about to create a timeline using diff_against quite often. Internally it uses django's model_to_dict which fires SQL queries for every ManyToMany field (twice - for every model related to the diff).

Describe the solution you'd like
I'd like to get rid of those SQL queries, as many to many relations are not supported so far.
Therefore I'd like to introspect the model and use the exclude argument to model_to_dict to avoid firing those queries.

Describe alternatives you've considered
Alternatively pass along the excluded fields to model_to_dict to at least give the caller of diff_against the opportunity to exclude many to many relations.

[sebashwa]

Duplicate of #776