$ git clone -n https://src.fedoraproject.org/rpms/rust-unchecked-index.git /var/lib/mock/f42-build-3251783-41222/root/chroot_tmpdir/scmroot/rust-unchecked-index
Cloning into '/var/lib/mock/f42-build-3251783-41222/root/chroot_tmpdir/scmroot/rust-unchecked-index'...
$ git fetch origin d4a4fbfc3db32e4b10367746a01f700ccb8d3ad7:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rust-unchecked-index
 * [new ref]         d4a4fbfc3db32e4b10367746a01f700ccb8d3ad7 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at d4a4fbf no longer needed