upgrade: add git branch check
For extensions / skins updated via git, the upgrade script should check whether the correct branch is checked out (i.e. if core is on REL1_33, the extensions / skins should be too, and not e.g. still on REL1_32.
For extensions / skins updated via git, the upgrade script should check whether the correct branch is checked out (i.e. if core is on REL1_33, the extensions / skins should be too, and not e.g. still on REL1_32.