send-email: use 'git hook run' for 'sendemail-validate'

send-email: use 'git hook run' for 'sendemail-validate'

Change the "sendmail-validate" hook to be run via the "git hook run"
wrapper instead of via a direct invocation.

This is the smallest possibly change to get "send-email" using "git
hook run". We still check the hook itself with "-x", and set a
"GIT_DIR" variable, both of which are asserted by our tests. We'll
need to get rid of this special behavior if we start running N hooks,
but for now let's be as close to bug-for-bug compatible as possible.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
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>