In the Repository Explorer (started with hgtk log on my Linux system) pick a changeset affecting the file in question. Right-click on the file name in the lower-left list. Choose "File History" from the menu.
You will now have narrowed the log to only that file. Click on the first revision you want to compare, then right-click on another revision. Choose "Diff with selected" from the menu. You will now see the differences between those two revisions.
(This is just the point-and-click version of Jason's answer.)