send-email: expose header information to git-send-email's sendemail-validate hook

send-email: expose header information to git-send-email's sendemail-validate hook

To allow further flexibility in the Git hook, the SMTP header
information of the email which git-send-email intends to send, is now
passed as the 2nd argument to the sendemail-validate hook.

As an example, this can be useful for acting upon keywords in the
subject or specific email addresses.

Cc: Luben Tuikov <redacted>
Cc: Junio C Hamano <redacted>
Cc: Ævar Arnfjörð Bjarmason <redacted>
Acked-by: Luben Tuikov <redacted>
Signed-off-by: Michael Strawbridge <redacted>
Signed-off-by: Junio C Hamano <redacted>