I looked at other makeProjectRelativeIfPossible() callers and they all seem ok except for one, FindInFiles._labelForScope(), which exposed a broader issue with "Find In...": you can right-click an untitled file and choose Find In, but it won't get any results since the file isn't in the FileIndexManager result.  I've spun this off as #4414 (for this sprint) so it doesn't hold up the pull request.