$ git clone -n https://src.fedoraproject.org/rpms/rust-cxx-build.git /var/lib/mock/f39-build-1538123-36388/root/chroot_tmpdir/scmroot/rust-cxx-build
Cloning into '/var/lib/mock/f39-build-1538123-36388/root/chroot_tmpdir/scmroot/rust-cxx-build'...
$ git fetch origin b8d275abc08a62dc49f0594124349cbad850b0bf:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/rust-cxx-build
 * [new ref]         b8d275abc08a62dc49f0594124349cbad850b0bf -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at b8d275a update to version 1.0.110 (rhbz#2246685)