Pushing the hook through the GitHub contents API dropped its mode to 100644.
assertHookMode caught it on the next commit, which is the gate working as
intended — a non-executable commit-msg would simply never run.
Also brings dev in line with the turn-limit fix already on main.
Issue: #116
User-Visible: no
The hook parsed the message path with basename, an external command. Where
it is missing from PATH the substitution yields an empty string, set -e does
not trip on it, and the MERGE_MSG guard silently stops working — a generated
merge commit would then be rejected for missing trailers it cannot have.
POSIX parameter expansion needs no external command and behaves the same in
sh, dash, bash and Git Bash.
Issue: #116
User-Visible: no