2 related issues with moving one file over an existing file

1) file X and Y exist.
mv X Y
was not reporting proper results. Was only reporting that X was deleted.
2) file dir1/X and dir2/X exist.
mv dir1/X dir2/X
similar issue. Old code reported 'dir1/X' was deleted. And nothing about dir2/X

I wrote some new unit test (directly interacting with DirectorySnapshot) which verified the failures, and then once the fix was in place, verified the correct behavior. See the new file tests/test_watchdog_directory_snapshot.py