It should be "is not an empty string" not "is not empty string". This
fixes wording originally introduced in ab9b31386b ("Documentation:
multi-head fetch.", 2005-08-24).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>