I have a footer that generates info about the diagram, like the date and user who created the image. I would like to include the git SHA. I need some sort of built-in function, probably a preprocessor function, that lets me envoke system commands. I'm thinking something like the %date() command, only where I pass in the command and arguments I want to be executed and I get back the text from it being run in the shell. Something like
%sys_call("git rev-parse --short=10 HEAD")