endpoint to get staging commits from a staging

[xmo-odoo]

/runbot_merge/stagings allows getting the stagings which contain a commit (or several), however:

1. the staging itself may not be ultra valuable, so would be a good idea to have an easy way to get the other commits for the staging
2. it doesn't filter on successful stagings

Also should probably be a regular http endpoint rather than a JSON one, now that the mergebot is on 17.0 and http repos should be able to return JSON or sth?