README: opt-in restack: Always interactive
README: opt-in restack: Always interactive

The `git-restack` script suggested as a means of
opt-in restacking left the addition of `-i` to users.
This version is better because without `-i`,
restack doesn't do anything.

Refs https://github.com/abhinav/restack/issues/30#issuecomment-1141445769
abhinav/restack,abhinav/restack