$ git clone -n https://src.fedoraproject.org/rpms/firefox.git /var/lib/mock/f39-build-1701991-41212/root/chroot_tmpdir/scmroot/firefox
Cloning into '/var/lib/mock/f39-build-1701991-41212/root/chroot_tmpdir/scmroot/firefox'...
$ git fetch origin e5fa239064142ee6fca21f177dd1c90660f8dd73:KOJI_FETCH_HEAD
From https://src.fedoraproject.org/rpms/firefox
 * [new ref]         e5fa239064142ee6fca21f177dd1c90660f8dd73 -> KOJI_FETCH_HEAD
$ git reset --hard KOJI_FETCH_HEAD
HEAD is now at e5fa239 merge