Use git plumbing commands to get the I3_VERSION
Use git plumbing commands to get the I3_VERSION

Fixes #3656

Airblader/i3-original,stapelberg/i3,stapelberg/i3,i3/i3,Airblader/i3-original,Airblader/i3,Airblader/i3,i3/i3,Airblader/i3,Airblader/i3-original,Airblader/i3,stapelberg/i3,i3/i3,i3/i3,stapelberg/i3,Airblader/i3-original,Airblader/i3-original