$ git clone -n https://src.fedoraproject.org/rpms/R-rhub.git /var/lib/mock/f42-build-side-1-3499335-45964/root/chroot_tmpdir/scmroot/R-rhub
Cloning into '/var/lib/mock/f42-build-side-1-3499335-45964/root/chroot_tmpdir/scmroot/R-rhub'...
$ git fetch origin ae2d7a59b32f4a6fcb9cf0d6030423b34d51295a:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-rhub
 * [new ref]         ae2d7a59b32f4a6fcb9cf0d6030423b34d51295a -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at ae2d7a5 R-maint-sig mass rebuild