Steps:
1. Make a new review request
2. Upload a diff
3. Change one of the files and make a new diff
4. Upload the new diff
RB shows both diffs in the history, but it should discard every diff except
the last one.
When I'm making a new review request, I scan through the diff before
publishing it. If for example I see the red squares of doom, I go back and
fix up the code and upload a new diff. The first diff is now useless,
there's no need to keep it around since I haven't published yet and no one
else has seen that diff.
