avoid whitespace on empty line in automatic usage message

avoid whitespace on empty line in automatic usage message

When outputting a usage message with a blank line in the
header, we would output a line with four spaces. Make this
truly a blank line.

This helps us remove trailing whitespace from a test vector.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>