What’s known issue? That if you screw up merging files (ie. blindly overwrite them) there might be problems? I agree with that. 
Anyway, who would have thought that creating some pacnew merger hook will cause these kind of problems…
@khandakershahi Boot from iso, mount / and edit /etc/shells - add zsh (or whatever you use) back in. Maybe even switching tty and login as root works. 