$ git clone -n https://src.fedoraproject.org/rpms/R-git2r.git /var/lib/mock/f43-build-side-110352-58907336-6569159/root/chroot_tmpdir/scmroot/R-git2r
Cloning into '/var/lib/mock/f43-build-side-110352-58907336-6569159/root/chroot_tmpdir/scmroot/R-git2r'...
$ git fetch origin dadb1ef5652de66ddff686787eb63bc5ff80429c:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-git2r
 * [new ref]         dadb1ef5652de66ddff686787eb63bc5ff80429c -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at dadb1ef R-maint-sig mass rebuild