Allow PRs to non-head bookmarks
Not being able to create PRs towards bookmarks unless they are heads makes it inconvenient to use bookmarks for branching.
Also see the discussion here: https://rhodecode-community.slack.com/archives/C19LYA9KQ/p1587567532080400
A fix for this problem that works for us in 4.18.0 is attached.
No data to display