Gerrit staging-approve should be able to read the success/fail comment from stdin

When we mark a build as failed with staging-approve, sometimes we'd like to include some error details which might be a few paragraphs or so.

This is cumbersome and error-prone to pass to staging-approve as a single command-line argument, so it would be nice if staging-approve could have an option to read the message from stdin.

Suggested interface is to accept - as a special meaning of "read from stdin" for the message. So, e.g., in the passing case we would usually do -m "No regressions!", while in the failing case we would usually do -m - and then pipe in a summary of the failures.