This has now been implemented. In order to compare two branches, you do:
- Check out one of the branches you want to compare with.
- Select the branch you want to compare with in the Git branch popup in the status bar in the bottom right of the IntelliJ window. A popup with some options is shown.
- Select the “Compare” option.
- This, by default, shows all the commits. You can also select the “Diff” tab to show the actual changes.
This is all described in much greater detail in one of their blog posts.