Timestamp based build: ambiguous argument

Created by: antonok-edm

When running pmbootstrap.py build linux-asus-grouper --arch=armhf --force, I get the following error

fatal: ambiguous argument 'origin': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

from the command git diff --name-only origin

My local repository was set up with the following commands:

git clone https://github.com/postmarketOS/pmbootstrap
git remote rename origin upstream
git remote add origin https://github.com/antonok-edm/pmbootstrap