Can not install CM 13.0 / 12.1 on galaxy s2 because of missing file_contexts
After some help from the irc-channel, the solution is (like described in https://web.archive.org/web/20161224194651/https://wiki.cyanogenmod.org/w/Install_CM_for_i9100) to use the boot.img from the cm13-zip (which seems in fact to be twrp) and flash it to the device with first booting in download mode (Power + Home + Down) and then running heimdall flash --KERNEL boot.img --no-reboot (KERNEL with capital letters). Afterwards, one can normaly install the zip using sideload.