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

* Prevent off-by-one error on comments on newly appended lines

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>

modules/git/diff.go

@@ -217,6 +217,8 @@ func CutDiffAroundLine(originalDiff io.Reader, line int64, old bool, numbersOfLi
 				} else {
 					otherLine++
 				}
+			case '\\':
+				// FIXME: handle `\ No newline at end of file`
 			default:
 				currentLine++
 				otherLine++