Why is it so hard to see code from 5 minutes ago?
I have had this same idea and would love to collaborate.  I can see tgis as either an extension to be added to existing IDEs or an enhancement to the Monaco Editor.I see it as a product named Multiverse Editing.  You start off coding in the prime universe.  Each Undo places a node on the current universe's timeline.  If you continue editing you are still on the prime timeline.  But you can click on any node in the timeline to go back, and editing from there branches to a new universe with its own unique timeline.  Each node represents either complete before and after code for the current timeline and a pointer to the root of one or more new universes.  you can select any two or three nodes and perform a merge which results in another new universe branching from the source (left) node of the merge.  A database will exist for each source file, each representing their own multiverse.  Source control events would become nodes on the timelines such as branches becoming a new universe and merges collapsing universes together.  Ideally the database would be in the SCP and not the editor, though that might get slow, nut that would preserve the complete edit history for a file.  Each node would have metadata where the developer could add notes in MD format, etc.