Mark conflicts properly in already-opened editors

This works around a sporadic bug wherein `DisplayBufferMarker` ids get scrambled in an already-opened `Editor`.

The easy way to fix this is just to make the markers that merge-conflicts creates non-persistent, so they don't collide with each other across editor reloading and rounds of conflict detection, and to clean them up when the conflict is resolved. There's a deeper bug here with marker serialization, though, which I may try to hunt down on its own, later.