$ git clone -n https://src.fedoraproject.org/rpms/rust-servo_arc.git /var/lib/mock/f39-build-1517127-35689/root/chroot_tmpdir/scmroot/rust-servo_arc
Cloning into '/var/lib/mock/f39-build-1517127-35689/root/chroot_tmpdir/scmroot/rust-servo_arc'...
$ git fetch origin 4f34f915b2c6396c1ea8f01cb678ca9254226d51:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rust-servo_arc
 * [new ref]         4f34f915b2c6396c1ea8f01cb678ca9254226d51 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 4f34f91 Update to 0.3.0