post-receive-email: fix accidental removal of a trailing space in signature line

post-receive-email: fix accidental removal of a trailing space in signature line

post-receive-email adds a signature to the end of emails in
generate_email_footer().  The signature was separated from the main email
body using the standard string "-- ". (see RFC 3676)

a6080a0 (War on whitespace, 2007-06-07) removed the trailing whitespace
from "-- ", leaving it as "--", which is not a correct signature
separator.

This patch restores the missing space, but does it in a way that will
not set off the trailing whitespace alarms.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>