[cardiffspaceman]

What if the tool does the committing, and automatically does the commits as deltas? Something like the idea that the 'save' is fairly similar to the undo-redo data rather than some optimized object graph or optimized relational database representing one iteration.

[wdfx]

Ardour has snapshots [1] which is a somewhat manual approach to this. I assume you can copy/paste parts of a session between snapshots as a "merge".

[1] http://manual.ardour.org/working-with-sessions/snapshots/

[rhizome]

Hydrogen also has a "save version" action that does something similar.