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>