Rename `latest_dev_green_build_commit` to `latest_successful_on_build`
We're in a class called Commits
already, so naming a method
latest_whatever_commit
was redundant. The new name also clarifies that
"green" is "successful", and we're specifically looking for commits that
also exist on Build (dev).
This also better matches the naming of its companion methods, latest
and latest_successful
.