Also post a comment in case base branch is wrong
This guides newcomers in how to smoothly handle the potentially scary
situation of having thousands of commits listed in a PR.
While CI shows the same, people might not even look at CI if the PR
looks botched.
(cherry picked from commit 369cfa02da)
This is needed such that in the next commit, we can re-use the same
version from a shell.nix, allowing people to have a guaranteed matching
nixfmt version.
(cherry picked from commit b33ac05d04)
