Allow changing of the compare_against setting globally, or on a per-project basis

My use case for this is that I have multiple projects around a repo where I have pull requests sent against different branches. This setting would make it ideal so that I can just add the setting in my individual projects without having to remember to do it every time.

Also, I can imagine this being useful for those who maybe use Git Flow and are comparing against a branch such as "develop" or "release".