Prevent off-by-one error on comments on newly appended lines (#18029) (#18035)

Prevent off-by-one error on comments on newly appended lines (#18029) (#18035)

* Prevent off-by-one error on comments on newly appended lines (#18029)

Backport #18029

There was a bug in CutDiffAroundLine whereby if a file without a terminal new line
has a patch which appends lines to it and a comment is placed on one of those lines
the comment diff will be a line out of place.

This fixes CutDiffAroundLine to simply ignore the missing terminal newline - however,
we should really improve this rendering to add a marker to say that there was a
previously missing terminal newline.

Fix #17875

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Apply suggestions from code review

Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>