repo/commit: Change most of this file to new code style

repo/commit: Change most of this file to new code style

I didn't touch everything since at least `commit_loose_object_trusted`
does this:

```
 out:
  if (G_UNLIKELY (error && *error))
    g_prefix_error (error, "Writing object %s.%s: ", checksum, ostree_object_type_to_string (objtype));
```

Which...it'd be interesting to make into an autocleanup. But for now just
keeping up with converting things bit by bit.

Closes: #761
Approved by: jlebon