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.
(cherry picked from commit 3b99066aa866e51e6a610716eaddfd1ea3645a67)
85bf170ff0