Things like signed commits would still use the full hash, so that would make tampering with that impossible. This solution would basically just make the UI bac

Things like signed commits would still use the full hash, so that would make tampering with that impossible. This solution would basically just make the UI backward-compatible while still requiring the complete modification of the internal to change the hash function. You'd still risk a collision if you refer to commits using a shortened hash outside of git but something tells me that you don't even need a vulnerability to take advantage of that if you have an attack vector. For instance github seems to use 7 hex digit in short hashes, this could probably be bruteforced relatively easily (be it for SHA-1 or SHA-256). To give you an idea I looked at the current bitcoin difficulty (which AFAIK uses two rounds of SHA-256 internally and works by bruteforcing hashes with a certain number of leaning zeroes) and the hashes look like this: 000000000000000000028048b31e42bd53d3b36da90d1a840ae695ec1a5ee738