add -i (Perl version): color header to match the C version

Both versions of `add -i` indent non-flat lists by five spaces. However
when using color the C version prints these spaces after the ANSI color
codes whereas the Perl version prints them before the color codes.
Change the Perl version to match the C version to allow for introducing
a test that verifies that both versions produce the exact same output.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-add--interactive.perl

@@ -483,10 +483,8 @@ sub list_and_choose {
 		my $last_lf = 0;
 
 		if ($opts->{HEADER}) {
-			if (!$opts->{LIST_FLAT}) {
-				print "     ";
-			}
-			print colored $header_color, "$opts->{HEADER}\n";
+			my $indent = $opts->{LIST_FLAT} ? "" : "     ";
+			print colored $header_color, "$indent$opts->{HEADER}\n";
 		}
 		for ($i = 0; $i < @stuff; $i++) {
 			my $chosen = $chosen[$i] ? '*' : ' ';