This was the only occurrence of "commitish" in the tree, but as the
log will reveal we've had others in the past. Fixes up code added in 00ad6e3182 ("git-p4: work with a detached head", 2015-11-21).
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Acked-by: Luke Diamand <luke@diamand.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>