send-email: check for repo before invoking hook

send-email: check for repo before invoking hook

Unless --no-validate is passed, send-email will invoke
$repo->repo_path() in its search for a validate hook regardless of
whether a Git repo is actually present.  Teach send-email to first check
for repo existence.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>