Fix incorrect diff expander for deletion of last lines in a file (#29501)

Fix incorrect diff expander for deletion of last lines in a file (#29501)

Fixes: https://github.com/go-gitea/gitea/issues/29498
I don't quite understand this code, but this change does seem to fix the
issue and I tested a number of diffs with it and saw no issue. The
function gets such value if last line is an addition:

```
  LastLeftIdx: (int) 0,
  LastRightIdx: (int) 47,
  LeftIdx: (int) 47,
  RightIdx: (int) 48,
```

If it's a deletion, it gets:

```
  LastLeftIdx: (int) 47,
  LastRightIdx: (int) 0,
  LeftIdx: (int) 48,
  RightIdx: (int) 47,
```

So I think it's correct to make this check respect both left and right
side.