Defacing git repositories doesn't even make sense. You won't mess with people's checkouts, its trivial to detect and identify the responsible party. It's the security equivalent of a child throwing a tantrum in their own room. You want to replace it? Everyone that comes after you and tries to push a change will immediately notice like opening the door to the proverbial child's room. You're busted and you've accomplished nothing.
You want to inject malicious code yourself? When it gets caught, or the file is inspected or reviewed you're busted.
This attack has the opportunity to get malicious code injected into a repository that will never show up in a PR, code review, or any existing checkout (so the senior developers that would notice the change most likely will never receive it). This is re-using an existing trusted and known good commit in your history, even the signature on it, to say "yeah this has always been here, this is perfectly safe and hasn't been modified since the author wrote it".
This is far more subtle, sneaky, and extremely valuable as an attack vector (and it gets more juicy, stay tuned) to get targeted vulnerabilities and backdoors into specific software. This isn't a novel attack method, as I mentioned the Linux kernel goes through a very rigorous process just to avoid this kind of attack.
Aside: You keep trying to use gibberish for your bad examples. You don't need to use garbage, that's the point I keep trying to hammer home to you. The added details can be from any generator and isn't constrained to living exclusively in comments. Garbage is what people use as examples for these attacks because its the easiest, and if you can demonstrate it for garbage then it works for any generator. With garbage you've made the point.
Back at the security issue. So now you have a poisoned repo that contains malicious code and is effectively undetectable through normal use. Meanwhile your production artifacts include the unaltered malicious code from the repository. It will remain unchanged and referenced until someone else creates _any_ change to the file you targeted (as once again git doesn't actually store diffs but whole files in a particular commit). That change might be something like a developer adding some print statements to try and diagnose why the CI system is failing.
When another change happens for that file the evidence mostly vanishes or at least is extremely obscured. There will be _some_ object in your repository that has the SHA-1 object, whether its the original or the malicious one depends entirely on when your checkout occurred.
On the receiving end your best case scenario is that the changed code doesn't work and causes weird bugs in your CI system that can't be reproduced in local checkouts and goes away magically as soon as anyone tries to diagnose it. This capability is worth STUPID amounts of money and I would be shocked if this isn't a technique used selectively in the wild by nation states.
SO how do you solve this problem?
* One of the inherent problems is that signatures don't actually cover the content of the commit. This is another regular complaint of git's behavior and would allow you to side-step this issue using the existing signing infrastructure. This is a bandaid but it's what most people argue for as it is significantly less of a lift than changing the hash function. If you're worried about the attack you just have to sign your commits and tags. If you sign your commits NOW without a change to git, you're still 100% vulnerable to this attack. and because the signatures will still be valid is likely to either make someone innocent look guilty of injecting a vulnerability, or will have audits look less closely at the code because it came from a trusted source causing more harm than good.
* Change out SHA-1 to something that isn't as vulnerable to collision attacks. The problem is collision attacks. Let me say that again, the core issue is with collision attacks. If you can create a chosen plaintext or chosen prefix attack the security guarantees of the git ledger goes away. You can't trust it. It needs to be replaced.
* If neither of those are options for you, your third option of protection is to adopt the Linux kernel policies. Releases are done directly from engineer's machines from a trusted known good repo that has patches added by hand by the most senior engineer.