$ git clone -n https://src.fedoraproject.org/rpms/R-bookdown.git /var/lib/mock/f42-build-side-1-3500330-45970/root/chroot_tmpdir/scmroot/R-bookdown
Cloning into '/var/lib/mock/f42-build-side-1-3500330-45970/root/chroot_tmpdir/scmroot/R-bookdown'...
$ git fetch origin 6df4797565ba216056e87a8c1bcf8009f585674e:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-bookdown
 * [new ref]         6df4797565ba216056e87a8c1bcf8009f585674e -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 6df4797 R-maint-sig mass rebuild