branch: change "--local" to "--list" in comment

There has never been a "git branch --local", this is just a typo for
"--list". Fixes a comment added in 23e714df91c (branch: roll
show_detached HEAD into regular ref_list, 2015-09-23).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
