Whatever happened to SHA-256 support in Git?

Furthermore, all currently feasible SHA-1 attacks are collision attacks - i.e. attacks in which the same person creates both the "good" commit and the "bad" commit. Signatures are primarily designed to deal with the case where the "good" and "bad" commits are created by different people (i.e. they are used to prove that a given commit was authored by the person identified in its metadata, and not an imposter). You can also use signatures to prove that some third party has reviewed the commit and believes it to be non-malicious, but to my understanding, that is not the typical use case (and, as you say, it is defeated by the collision attack anyway).