Notify '*' pathspec correctly when diffing

I also moved all tests related to notifying in their own file.
Add 2 failing tests related to '.' handling

No files matched. It fails in `workdir_iterator__update_entry`, as it thinks there is a "." (respectively "./subdir") prefix.

I don't understand iterators well enough to propose a correct fix for now (if there needs one). So far, here is what I thought of:
- Should we "compact" the passed pathspecs, in order to remove the "." and ".."?
- Should we have a custom `prefixcomp` function for iterators, that handles "." and ".." differently?
Add file filtering plugin capabilities to diff