$ git clone -n https://src.fedoraproject.org/rpms/xstream.git /var/lib/mock/f38-build-side-42-init-devel-671582-26679/root/chroot_tmpdir/scmroot/xstream
Cloning into '/var/lib/mock/f38-build-side-42-init-devel-671582-26679/root/chroot_tmpdir/scmroot/xstream'...
$ git fetch origin ac4b6e4993287084deb2b22a54f64719b22c0508:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/xstream
 * [new ref]         ac4b6e4993287084deb2b22a54f64719b22c0508 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at ac4b6e4 Depend on compat versions of activation and XML bind (by @mkoncek)