evolve: stop preserving all of {extras} for uninterrupted evolve operations

I doubt the wisdom of doing this, but the case of completing interrupted evolves
does this, as do core commands for amend, graft, and rebase in hg ce9696193175,
bb14a81ce647, and b698abf971e7.

Note the obslog order (and DAG display) change in test-evolve-content-divergent-relocation.t
around line 131.  I have no idea why that would happen, or if it's an indicator
of something subtle going on.
8 jobs for topic/default/evolve-continue-extras in 18 minutes (queued for 3 seconds)