Pull request event handlers block creation of pull request when merging a branch in a fork to the origin repository branch

This happens with block out of date pull requests and pull request policy advice.

This is due to the fact that we only calculate the merge base for the current repository and not cross repository.

com.atlassian.stash.internal.scm.git.pull.CommonAncestorPullRequestAutoMergeStrategy in the Bitbucket source code shows how we may want to handle this.