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>

t/t1502-rev-parse-parseopt.sh

@@ -5,7 +5,7 @@ test_description='test git rev-parse --parseopt'
 
 cat > expect.err <<EOF
 usage: some-command [options] <args>...
-    
+
     some-command does foo and bar!
 
     -h, --help            show the help