Expanding a collapsed chunk in an interdiff could fail with a `TypeError` comparing an `int` to a `str`. The same failure could occur when computing the line ranges shown for a comment in review e-mail. Diff lines store a missing line number as an empty string. Interdiffs filter out unrelated opcodes and merge the results back into "equal" chunks, which can leave one side of a chunk with fewer lines than the other. The lines at the end of such a chunk have an empty line number on the shorter side. Two places assumed the last line of a chunk always had a line number for both sides. `DiffRenderer.make_context()` used it to filter the function/class headers down to the newly expanded range, and `get_displayed_diff_line_ranges()` used it to cap the end of a comment's displayed range. Both compared or added it to an int, which failed whenever the last line was missing that side. `_get_last_header_in_chunks_before_line()` already had a nested `find_last_line_numbers()` that scanned backwards for the last line with a number on each side, which described this exact case. I've moved that into a module-level function in `diffutils`, and both of the above call it. Testing Done: - Reproduced the chunk expansion error from a production traceback in a unit test, and verified the fix. - Verified the new `get_displayed_diff_line_ranges()` tests failed before the fix, with the same error. - Ran all unit tests.