$ git clone -n https://src.fedoraproject.org/rpms/rust-boxcar.git /var/lib/mock/f42-build-side-1-3534272-46548/root/chroot_tmpdir/scmroot/rust-boxcar
Cloning into '/var/lib/mock/f42-build-side-1-3534272-46548/root/chroot_tmpdir/scmroot/rust-boxcar'...
$ git fetch origin 2b1460b1fac220feff400176ab7879244c2013d8:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rust-boxcar
 * [new ref]         2b1460b1fac220feff400176ab7879244c2013d8 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 2b1460b Update to 0.2.13 (close RHBZ#2372348)