Phototoshop has apparently had this feature for some time. They call it "non-linear history". You can go back to earlier revisions, try new things starting from

Phototoshop has apparently had this feature for some time. They call it "non-linear history". You can go back to earlier revisions, try new things starting from there, then if you don't like it, erase that history path and go back to your first one. You can also "snapshot" your history so points on it have intuitive labels. They don't display it as a tree, but it's logically identical -- just flattened out into a list form with the inactive branches grayed out.