filter_lint_by_diff fails on deleted files

[tautschnig]

I'm seeing the below assertion on #166 and others - and I'm led to believe this is happening whenever files are deleted.

The linter output is below (empty output means no errors).
Traceback (most recent call last):
File "scripts/filter_lint_by_diff.py", line 18, in
assert filename.startswith("b/")
AssertionError

Best,
Michael

[smowton]

Fix in #564