$ git clone -n https://github.com/fedora-riscv/firefox.git /var/lib/mock/f39-build-1810578-41828/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f39-build-1810578-41828/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin 26bfaf9ce14695820abcbaae982a129c6641c3e9:KOJI_FETCH_HEAD
From https://github.com/fedora-riscv/firefox
 * [new ref]         26bfaf9ce14695820abcbaae982a129c6641c3e9 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at 26bfaf9 Rebuild for f39