I remember that I used to be able compare a file between two commits. The feature is still there, but the commit select list does not show up after I click on it. It used to be working so I wonder if it is caused by a recent update.
For your information, the feature is accessed by directly choosing "Diff" on a upper right corner when viewing a file. After clicking on Diff, it prompts me by asking me to "Diff from" some commit and some commit. But when I clicked on those some commit's nothing happens though I do have a series commits to choose from.
I appreciate your service a lot, it will be even better if you can fix this problem and get this awesome feature back online again :)