Comment by gsu2

I might be mistaken, but wouldn't a git signature already be signing trusted things (i.e. the person making the original signature is trusted), making any attack enabled by the input hash function a second preimage attack (i.e. an attacker onky knows the trusted input, not anything private like the signing key)?

Hash collisions mean you can't trust signatures from _untrusted_ sources, but git signatures don't seem to fit that situation.