When redoing a history entry, its `is_modified` flag is not
reflected to the history state tracker. So GtkTextBuffers may
expose a modified=FALSE status, despite a change was actually
applied to the buffer.
For the undo case, an `is_modified_set` flag was set on the last
entry of the undo queue when a change of the modified state of
the history is requested. This commit does the same on the first
entry of the redo queue.
Closes#5777
b4977decc1