What version are you running?
2.0.18 (also tried 2.5.1.1) (rbtools 0.7.5)
What's the URL of the page containing the problem?
/r/N/diff
What steps will reproduce the problem?

Using git, edit file A, commit, rbt post, and publish
Rebase file A, getting new changes
Modify one of the sections that was added to the file during the rebase
Amend commit
rbt post -r REVIEW_ID

What is the expected output? What do you see instead?
The interdiff between revisions 1 and 2 should show the additional modifications you made during step 3, but intead it says "This file contains only whitespace changes."   Occasionally after rebase, you need to make additional changes to support the new code that was pulled in, and it is useful to be able to see these changes in the diff viewer.
What operating system are you using? What browser?
Linux, Chrome 46
Please provide any additional information below.
The interdiff capability in the diff viewer is very useful in reviewboard, but it is not obvious what the expected behavior is when the base revision changes like this.  At the very least, the current behavior should be documented in the reviewboard documentation.  I can understand the desire to filter out the new file contents that were pulled in during the rebase, but if part of those new lines were also changed as part of the new commit being reviewed, one would expect them to be shown in the interdiff (since that is what changed between diff revisions 1 and 2).