*NOTE: Do not post confidential information in this bug report.*

What's the URL of the page containing the problem?
/r/##/

What steps will reproduce the problem?
1. Open a review request
2. Click on "Update Attached Diff"
3. The "Base directory" is blank

It would be nice if it automatically populated the base directory from the
most recent diff in that review request.  The base path typically won't
change between iterations.