Well, apparently Eclipse has more functionality for this specific purpose. That may be enough for you to completely dismiss IDEA in a fit of self-righteous prejudice, but that doesn't mean IDEA isn't a better product overall.
You can choose specific files to commit or update in IDEA too, but I guess that's no good either if it doesn't happen exactly where you want it to.
Why do you want a visual merge even when there are no conflicts though? You could just edit out everything that shouldn't be committed, and then commit.