Revision browser shouldn't show unsaved changes when there are no changes

[wlach]

Currently the iodide revision browser shows an unsaved changes when the notebook is already fully up to date:

We should only show this item when there are changes not yet synchronized with the server (i.e. we actually have a difference to show, not just "no difference").

The logic to display this item in the list is here:

iodide/src/editor/components/modals/revision-list.jsx

Line 52 in e361ed0

<ListItemText primary="Unsaved Changes" />

The logic to calculate the difference between two revisions is here, it might provide some hints on how to implement this feature:

https://github.com/iodide-project/iodide/blob/e361ed03c6ae04c015a1ba19e4f51dd012870f4e/src/editor/components/modals/revision-diff.jsx