Track build status to pull request, not just commit id

Track build status to pull request, not just commit id. The extension of API call would make builds more robust. If a build fails due to a network or system build issue, not just the build objects, the failure check would be much more useful.

Build status has no way to track a build to a pull request. A build is tracking to a single git commit. There should be the ability to track a build status to a pull request and commit id.

From https://developer.atlassian.com/server/bitbucket/how-tos/updating-build-status-for-commits/

curl -u username:password -H "Content-Type: application/json" -X POST http://localhost:7990/bitbucket/rest/build-status/1.0/commits/9e72f04322c4a1f240e0b3158c67c3c19cdd16e7 -d @build0.json

Now:

{
	"state": "SUCCESSFUL",
	"key": "REPO-MASTER",
	"name": "REPO-MASTER-42",
	"url": "https://bamboo.example.com/browse/REPO-MASTER-42",
	"description": "Changes by John Doe"
}

Proposed:

{
	"state": "SUCCESSFUL",
	"key": "REPO-MASTER",
	"name": "REPO-MASTER-42",
	"url": "https://bamboo.example.com/browse/REPO-MASTER-42",
	"pullrequest": "PR-321"
	"description": "Changes by John Doe"
}

Bitbucket would also need to have the kickoff of the build to have the pull request name in the API call that started the build. This request is due to all of the other Merge Checks are pull request, like reviewers, only modifiers. With the pull request being attached to a commit only, this can lead to confusion.