Previously, diff fragment comments from interdiffs were sometimes

resulting in a blank diff fragment. This was because of the diff

fragment viewer relying on the 'total_line_count' value in the

filediff/interfilediff line counts. However, this value is not always

guaranteed to have a value. Now we specifically calculate the last

line in the filediff/interfilediff to avoid this issue.

Ran unit tests.
Verified that https://reviews.reviewboard.org/r/6815/#comment19874

does not appear as a blank diff with the patch applied. To do this I

saved and uploaded all the diffs in order and created an issue on the

same lines in the same interdiff.