I think long is fine if that's necessary to get a rough idea of what the change is about across. Discerning in the choice of what information to include is perhaps a better way to put it. In those terms we should consider how likely it is for prompts to be immediately useful information when browsing the commit history, and whether it can't instead be reduced to an informative summary.

As far as specifications for changes go, the diff itself is as good a spec as it gets. It unambiguously describes the exact change that was made. The prompt you give a chatbot to make that change is IMO something else. Maybe it's more fair to call that a specification of the work you wanted it to perform.