$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f41-build-side-1-3049598-37900/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f41-build-side-1-3049598-37900/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin 3e87328081ff6185b2ea61e625eaa79d8486bd11:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         3e87328081ff6185b2ea61e625eaa79d8486bd11 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 3e87328 Enabled PGO on x86_64