new upgrade bot loses most useful changelog: binderhub,

[minrk]

binderhub and repo2docker bumps include the git hash, and henchbot created automatic GitHub compare links to show the changes (extracted hash from image/chart tag and put https://github.com/repo/compare/$before...$after in the body).

It would be handy if our new bot could preserve that functionality, since it's the most relevant change set to have a link to from the PRs.

[manics]

I just started looking into this, I think the easiest way is to pull out the short commit SHA from the image name, e.g. 7580a30..0345a86

-      build_image: quay.io/jupyterhub/repo2docker:2022.02.0-88.g7580a30
+      build_image: quay.io/jupyterhub/repo2docker:2022.02.0-90.g0345a86

and modify https://github.com/executablebooks/github-activity to

• (edit: this may not be needed) pull out the list of PRs between commits (I've got an old PR in [ENH]: Filter PRs by checking for commits in branch executablebooks/github-activity#54 that almost does that)
• add some output flags to only output the list of PRs

[manics]

I decided it'd be easier to write a script: #2348