$ git clone -n https://src.fedoraproject.org/rpms/R-bit64.git /var/lib/mock/f38-build-723873-99307/root/chroot_tmpdir/scmroot/R-bit64
Cloning into '/var/lib/mock/f38-build-723873-99307/root/chroot_tmpdir/scmroot/R-bit64'...
$ git fetch origin b0afc77cd9bccc257c9b9b76b9256eb7748fd8b6:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/R-bit64
 * [new ref]         b0afc77cd9bccc257c9b9b76b9256eb7748fd8b6 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b0afc77 R-maint-sig mass rebuild