`pipelines:diff` breaks down when the latest release is not a deploy

[joshwlewis]

It looks like we only attempt to fetch the latest release for each app here. If the latest release is not a deploy, we'll fail to diff that app.

⬢ app-a was not compared to ⬢ app-b as ⬢ app-b does not have any releases

[joshwlewis]

Perhaps we could fetch the last few releases, find the first one with a commit sha and perform the diff on that.