Improve Diff functionality in Crucible and FishEye

Premise for Crucible:
Commit a file 3x, each time adding some lines of "code", generating three revisions on the repository: r1, r2 and r3.

In Crucible Reviews, if you:

1. Click on "Create Review"
2. Select "Search for Files"
3. Select, for example, r1 and r3
4. Click on "Start Review"
5. Click on the file listed on the left hand side
6. All three revisions are displayed.

According to our development team, this is the default behaviour of Crucible, because in the general sense, diffs are cumulative. For a sequence of three revisions r1, r2, and r3, the diff from r1 to r3 includes the diff from r1 to r2 and the diff from r2 to r3, since the revision r3 is based on those two diffs.

This is something we can improve in the UI however, as this has caused a good bit of confusion to some users. For example, greying out the revisions corresponding to an intermediate diff would help identifying which pair of revisions are being "diff" at the moment.

Any other suggestions on how to improve this are welcome.