$ git clone -n https://src.fedoraproject.org/rpms/R-R.devices.git /var/lib/mock/f42-build-side-1-3500779-45971/root/chroot_tmpdir/scmroot/R-R.devices
Cloning into '/var/lib/mock/f42-build-side-1-3500779-45971/root/chroot_tmpdir/scmroot/R-R.devices'...
$ git fetch origin aeaa7c8b56006ddc84580b1bbb49e4c7c3e06be3:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-R.devices
 * [new ref]         aeaa7c8b56006ddc84580b1bbb49e4c7c3e06be3 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at aeaa7c8 R-maint-sig mass rebuild