quick run mode: ignore indirect dependencies

[timbertson]

for quick feedback, you might not want every file that transitively depends on your code to be re-run every time a change is made.

so we should add a flag to filter the "changed" fileset to only those that have been changed themselves (modified tests), or files that themselves import the changed code (one or two test files, presumably).

There's also the possibility of restricting tested files to only those that include a changed file's name -> i.e. if "foo.py" changes, only files that include "foo" will be run.