x/exp/cmd/gorelease: decide on report and diagnostic format

[jayconrod]

gorelease's output format is a bit messy. It's mostly inherited from apidiff without much additional thought.

Now that we have a more complete set of diagnostics, we should think about exactly what the output should look like.

I don't have a specific proposal yet; this issue is mostly a placeholder.

cc @jadekler

[gopherbot]

Change https://golang.org/cl/332375 mentions this issue: cmd/gorelease: change report output format

[gopherbot]

Change https://golang.org/cl/333370 mentions this issue: cmd/gorelease: remove empty summaries for comparisons