Improve setup for branches for MW releases (REL1_35 etc.)

[thaider]

Some users rely on widespread branching convention for git repositories of MediaWiki skins and extensions. The current solution is to create branches at the time when new MediaWiki versions are released. It would be good to update the branches with bug fixes and security updates. As Tweeki is going to be supporting LTS versions of MediaWiki, REL1_32 through REL1_34 would be mere aliases for REL1_31. It would be nice to have some sort of automated setup to merge and push changes in the dependent branches.