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.

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.