avoid crashing if cell id doesnt exist when diffing notebooks (#1148)

* avoid crashing if cell id doesnt exist when diffing notebooks

* Lint the code

Co-authored-by: Frédéric Collonval <[email protected]>

Author: DeanLight

jupyterlab_git/git.py

@@ -380,7 +380,7 @@ def read_notebook(content):
         # TODO Fix this in nbdime
         def remove_cell_ids(nb):
             for cell in nb.cells:
-                del cell["id"]
+                cell.pop("id", None)
             return nb
 
         current_loop = tornado.ioloop.IOLoop.current()