git hook run: add an --ignore-missing flag

git hook run: add an --ignore-missing flag

For certain one-shot hooks we'd like to optimistically run them, and
not complain if they don't exist.

This was already supported by the underlying hook.c library, but had
not been exposed via "git hook run". The command version of this will
be used by send-email in a subsequent commit.

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