$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f40-build-1915306-25086/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f40-build-1915306-25086/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin 8e79cd605a6f5e0ea4d4339622dae94c347bbd4d:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         8e79cd605a6f5e0ea4d4339622dae94c347bbd4d -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 8e79cd6 new sources