That heuristic would work most of the time. But it rests on the assumption that "if commit A has an older timestamp than commit B, then any user who saw commit

That heuristic would work most of the time. But it rests on the assumption that "if commit A has an older timestamp than commit B, then any user who saw commit B must have also seen commit A", which is not reliable in a distributed version control system. It seems safer to just explicitly tell the user when they're trying to work with an ambiguous hash.