Test that any changes to the index will mark the index as dirty.  Also
ensure that when we initialize a new index, read the index contents
from disk, or write the index contents to disk that we reset the dirty
flag to zero.  Further ensure that an unforced read with dirty contents
(when the on-disk index has not changed) does _not_ reset the dirty
flag as we have not updated the contents of our index and our unsaved
contents remain intact.