get_app_pending_mutations() is responsible for looking up the

evolutions, stored signature, and current signature for an app, and then

filtering a list of mutations based on the differences between those

signatures. This requires lookups to be performed, which can be slow and

may not reflect the state we want used for the computations.
This change allows the project signatures and the list of mutations to

be provided by the caller instead of automatically looking them up.
It also adds unit tests for this function, since those didn't exist

before.

Unit tests pass for all versions of Python and Django.