Restore the worktree after a failure

```git-check-assertions
run ./test/git-check-assertions.bats
assert_success
```

bin/git-check-assertions

@@ -84,6 +84,7 @@ for commit_hash in "${commits[@]}"; do
     printf '%s\n' "$block" | sed 's/^/> /'
     if ! bash -euo pipefail -c "$block"; then
       echo "git-check-assertions block failed in $commit_hash" >&2
+      git restore -q .
       echo "Returning to $orig_ref"
       git checkout -q "$orig_ref"
       exit 1