The rollback cells blind the pre-pull syntax check so the broken release reaches the post-pull
guard. That check now reads the target in one git cat-file --batch (c8291c00b6b), so the git
shim also answers the broken file's batch request with HEAD's copy. The parked-branch cell
kills the checkout that moves the tree; since review O2 the -B form names the resolved commit,
so -B main anywhere is the move, not only a last argument of main.
a931796ce2