$ git clone -n https://src.fedoraproject.org/forks/vondruch/rpms/rubygem-rack.git /var/lib/mock/f43-build-61006428-6587583/root/chroot_tmpdir/scmroot/rubygem-rack
Cloning into '/var/lib/mock/f43-build-61006428-6587583/root/chroot_tmpdir/scmroot/rubygem-rack'...
$ git fetch origin daab1ecc14ddffe59402a4c139f641e7050428c8:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/forks/vondruch/rpms/rubygem-rack
 * [new ref]         daab1ecc14ddffe59402a4c139f641e7050428c8 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at daab1ec Update to Rack 3.1.16