Message header lost on commit error

### Summary

When you try to commit a migration which has a `--! Message:` header in `current.sql`, and that migration fails for some reason, e.g. syntax error, `current.sql` is restored, but the `--! Message:` is lost.

### Steps to reproduce

Have contents of `current.sql` as such:
```sql
--! Message: test-migration

DROP TABLE IF EXISTS app_public.table_name CASCADE;
CREATE TABLE app_public.table_name(
id TEXT PRIMARY KEY,
title TEXT, --this comma causes an error
);
```
run the `commit` command and get an error like this:

### Expected results

Have contents of `current.sql` the same as before failure:
```sql
--! Message: test-migration

DROP TABLE IF EXISTS app_public.table_name CASCADE;
CREATE TABLE app_public.table_name(
id TEXT PRIMARY KEY,
title TEXT, --this comma causes an error
);
```

### Actual results

Actual contents of `current.sql` are missing the header:
```sql
DROP TABLE IF EXISTS app_public.table_name CASCADE;
CREATE TABLE app_public.table_name(
id TEXT PRIMARY KEY,
title TEXT, --this comma causes an error
);
```

### Additional context

graphile-migrate version 1.0.2.

### Possible Solution

Since there is a log before that like
```text
graphile-migrate[shadow]: Running migration '000002-test-migration.sql'
```

It should be possible to get the message value at least from filename.