Caught by @justusbunsi
An old bug from #6488
In `pullrequest_targetbranch_change`, the `data` might be empty, because
`UpdatePullRequestTarget` may respond `http.StatusNoContent`.
And the old code's `$branchTarget.text(data.base_branch);` doesn't make
sense, because in the end, the page will be always reloaded.
So, just remove the `$branchTarget.text(data.base_branch);`, everything
should be fine.
