a8n: Creating pull requests on Bitbucket Server fails due to wrong refname

Created by: mrnugget

Right now creating PRs on Bitbucket Server fails because of this error message:

 Bitbucket API HTTP error: code=404 url="https://bitbucket.sgdev.org/rest/api/1.0/projects/SOUR/repos/automation-testing/pull-requests" body="{"errors":[{"context":null,"message":"Repository \"automation-testing\" of project with key \"SOUR\" has no branch \"refs/heads/refs/heads/master\"","exceptionName":"com.atlassian.bitbucket.commit.NoSuchCommitException"}]}"

Note the refs/heads/refs/heads/master.

That's probably due to my changes in https://github.com/sourcegraph/sourcegraph/commit/49b07982a9fbce4c7864dfa0b95340d8f02ff0fb