This *may* now return Add or Delete Changes as appropriate. All I know
for sure is that it compiles.
I had hoped to avoid maintaining my own state about the content of the
directory tree, and rely on git to check what was changed. But I can't;
I need to know about new and deleted subdirectories to add them to the
watch list, and git doesn't deal with (empty) directories.
So, wrote all the code to scan directories, remember their past contents,
compare with current contents, generate appropriate Change events, and
update bookkeeping info appropriately.
