fix bug in git notebook hook where deleted files would cause an error

Author: nilsceberg

git-hooks/pre-commit

@@ -17,6 +17,10 @@ for filename in staged_files:
     if filename.endswith(".ipynb"):
         # Load notebook as it looks in the staging area using 'git show :filename' (note the colon)
         completed_process = subprocess.run(["git", "show", ":" + filename], capture_output=True)
+        if completed_process.returncode != 0:
+            # This probably means that the file is staged for deletion
+            continue
+
         notebook = json.loads(completed_process.stdout.decode("utf-8"))
 
         # If any cell contains output, warn and set abort flag