You need enough leeway in the first commit though. As this is a collision attack, you need to be able to tweak both commits in order to get their hashes to collide.

The mangling in the first commit is probably going to look mighty-suspicious. You might be able to handle this if you get to mangle e.g. code comments. In any case, you need something similar like PDFs malleability where you can change one document without the change being noticeable.