I would have liked sha-256 truncated to 40 characters (by analogy to sha-512/256 that'd be sha-256/160). Cryptographically that's probably fine. But 160 bits is not a lot. And probably fine doesn't tend to inspire confidence in the field of cryptography. It's not a very well studied scheme

An advantage of a hash with a different length is that a full length sha1 commit hash and a full-length sha256 commit hash can't be confused for each other