[Bf-python] Add a blender2.8 branch for addons git repo

[Bastien Montagne]

Hey fellow scripters,

I think it’s time to have a '2.8' branch for addons repository too. I 
can think to two main usages for it at least:

* Keep a few 'key' addons working(-ish) with blender2.8 branch - 
especially with redesign of active/selected objects etc., could help to 
catch early crucial issues in new API.
* Allow to start using 2.8-only new features (thinking about asset 
engines here for near future, but quite sure there will be more cases too).

that blender2.8 addons branch would then be automatically 'linked' into 
main repo’s blender2.8 branch (will require git 1.8.2 at least, but that 
one is already years old, so should not be an issue). What do you say?

Best regards,
Bastien