Track Before Sha for Bitbucket Server

[bradrydzewski]

the Bitbucket Server push payload has the target sha, which we should use as the before sha. Example value in payload here:

go-scm/scm/driver/bitbucket/testdata/webhooks/push.json

Line 21 in 1e77204

"hash": "40e7580cf11311d84a6e5e97e2cbba6df1675750",