Yeah, I just emerged from the Git code myself ;-) So you're correct that GPG-signing commits (but not tags) prevents collisions in commit objects. The problem

Yeah, I just emerged from the Git code myself ;-) So you're correct that GPG-signing commits (but not tags) prevents collisions in commit objects. The problem though is that a commit ultimately contains a SHA-1 hash of a tree object, so now the concern is someone generating colliding tree objects. Edit: fortunately, the format of tree objects looks pretty rigid. I feel somewhat reassured, but only somewhat.