$ git clone -n https://src.fedoraproject.org/rpms/R-repr.git /var/lib/mock/f42-build-side-1-3498660-45963/root/chroot_tmpdir/scmroot/R-repr
Cloning into '/var/lib/mock/f42-build-side-1-3498660-45963/root/chroot_tmpdir/scmroot/R-repr'...
$ git fetch origin b542cff431d16296d29aee57a21ea26e09d8027a:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-repr
 * [new ref]         b542cff431d16296d29aee57a21ea26e09d8027a -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b542cff R-maint-sig mass rebuild