post-receive-email: ensure sent messages are not empty

post-receive-email: ensure sent messages are not empty

Changes the logic in the script to determine whether an email message
will be sent before invoking the send_mail() function; otherwise, if
the logic determines that a message will not be sent, send_mail() will
cause an empty email to be sent. In addition, ensures that if multiple
refs are updated and a message cannot be sent for one of them,
the others are still processed normally.

Signed-off-by: Kevin P. Fleming <kpfleming@digium.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>