GitBucketだけじゃなくてBitBucketとかGitHubが元の仕様なんでしょうが。
Githubだと仕様はこれ
Webhooks | GitHub API
https://developer.github.com/webhooks/
payload={"pusher":{"name":"ymko","email":"ymko@xxxxxxx"},"ref":"refs/heads/dev","commits":[{"id":"f4c3e82e39ab5252422368c26a2876957469b6cf","message":"aaa\n","timestamp":"Sun Jan 11 20:27:36 GMT 2015","url":"http://xxxxxxxx:xxx/pigs/suzaki-nishi/commit/f4c3e82e39ab5252422368c26a2876957469b6cf","added":[],"removed":[],"modified":["20150106.txt"],"author":{"name":"ymko","email":"ymko@xxxxxxxx"}}],"repository":{"name":"suzaki-nishi","url":"http://xxxxxxxxxxxxxxx:xxx/git/pigs/suzaki-nishi.git","description":"洲崎西","watchers":0,"forks":0,"private":false,"owner":{"name":"pigs","email":"pigs@devnull"}}}
見てる人がいた。
Bitbucket の service hook で POST されてくるパラメータ例 - メモ
http://elm-arata.hatenablog.com/entry/2013/05/23/190945