[pshc]

It doesn't sound possible for all but the most trivial diffs. An attacker would have to guess not only the exact commit text, but also its timestamp. And the only attack scenario I can conceive of seems pretty silly. Maybe I'm missing something...

Alice clones an open source git repo, commits one secret change where she edits a config file's default password to her own secret password (a bad practice), and then publishes the new hash in public for some reason (build info?). Mallory would have to (a) know that exactly this happened, (b) guess the commit message, (c) guess the commit's timestamp to the second (or within a few seconds), and (d) preimage-attack her password.

And the preimage attack must pierce git's Merkle tree, which sounds downright impossible. (Unless Mallory is just bruteforcing, in which case a strong password is enough.)