Bitbucket Stash driver doesn't handle event `pr:from_ref_updated` (new commits / force push)

[raphendyr]

It seems to me that Drone ignores changes to from ref, i.e., new commits to PR and thus a new build is not started for the PR.

If I have understood correctly, this part is handled in stash/webhook.go and possibly just adding pr:from_ref_updated might fix this.

go-scm/scm/driver/stash/webhook.go

Lines 45 to 47 in 3a08005

	case "pr:opened", "pr:declined", "pr:merged":
	hook, err = s.parsePullRequest(data)
	}

[abdennour]

@raphendyr open pr please and link it with this issue like this: #116