Keep a squash's trailers when its files were touched up

The stale-SQUASH_MSG guard required a staged file to be byte-identical to a squashed commit's version, so fixing a typo in the squashed file before committing stripped the trailer git itself seeded, a regression from main. Match on paths instead: SQUASH_MSG is current when a staged path is one its commits changed. Staged work on other paths still inherits nothing.