Show build results in arc land
Fixes T4809. When landing a revision, check for a (non-manual) buildable of the current diff. If we find one, check its status:
- If it passed, print out a message to inform the user that we checked.
- If it failed or is still building, print out details about the issue and require a confirmation to continue.
- Just ignore other cases.
- Ran arc land on a revision with no buildable, a passing buildable, a failed buildable, and a building buildable for the current diff.
- Got sensible output / prompts.
Reviewed By: btrahan
Maniphest Tasks: T4809
Differential Revision: https://secure.phabricator.com/D8801