Missing `exec`s

[zmitchell]

Steps to reproduce:

$ proctrace record -o demo.log -- ./demo_script.sh
$ proctrace render -i demo.log -d by-process

You'll see two processes that fork without an exec.
I'm pretty confident these are the printf calls in demo_script.sh.
It's possible that somehow the shell builtin printf is getting called,
but I tried to avoid that by calling command -v printf directly.