modification time of the most recent save is different, this entry is
obsolete.
+An entry (t . 0) means means the buffer was previously unmodified but
+its time stamp was unknown because it was not associated with a file.
+An entry (t . -1) is similar, except that it means the buffer's visited
+file did not exist.
+
An entry (nil PROPERTY VALUE BEG . END) indicates that a text property
was modified between BEG and END. PROPERTY is the property name,
and VALUE is the old value.