Exec command don't trigger prompt function update

[jcubic]

Issue summary

If you have dynamic prompt and a command that change that prompt (e.g. using a variable), and when you exec() that command, the prompt is not updating.

Expected behavior

Prompt is updated

Actual behavior

Prompt is not updated

Steps to reproduce

1. Open demo https://codepen.io/jcubic/pen/KKYBWog?editors=0010
2. Wait for the exec to run

Browser and OS

N/A

Additional notes

The prompt updates when you run the same command by hand

It also updates when command is async, the same as with #926