- Fetch the ACKs only at sign-off time. This makes sure that any last-minute ACKs are included (fixes #16200)
- Show a list of ACKs that will be included and their author before signing off, and warn if there are none

There's a slight change to the merge commit format—before it was
ACKs for commit 88884c:
(list of ACKs, could be empty)
now it is
ACKs for top commit:
jnewbery:
ACK 5ebc6b0eb
... (list of ACKs cannot be empty)
or
Top commit has no ACKs.
I don't think there's a reason to have the abbreviated commit ID there, after all the full commit id is already in the beginning of the merge commit message, and at least the abbreviated one is in every single ACK message.

