`squash` discards history of all sub-namespaces

[pchiusano]

squash new old where new is ahead of old should keep all the old history and just create (at most) a single step for any updated subnamespace.

Instead it appears to discard all history of all subnamespaces of old, leading to really bad LCAs later.

Step 1 is just a transcript to reproduce this.

[aryairani]

IIRC this would be a new command. I don't think any special knowledge is needed to implement this, but I'm happy to discuss or review.