The script failed in the Prow job with:
fatal: Not a valid object name origin/master
That came from "git merge-base origin/master HEAD", which isn't needed because
the Prow job intentionally sets -r explicitly.
Now the fallback is only tried if needed. While at it, the informational
messages get enhanced.
Pinning apidiff to a specific version shouldn't be necessary because, if past
experience in klog holds true, the latest version just works. This way we don't
have to remember to bump up the revision.
The downside of using "latest" is that a compromise of that version would give
an attacker the ability to run code in the CI and on developer machines.
apidiff can be invoked for a single internal package or iterate over
everything, including staging. The base to compare against can be specified via
-r (similar to verify-golangci-lint.sh), with the default the base revision on
master (similar to -a in verify-golangci-lint.sh).
