gitweb: Fix "Use of uninitialized value" warning in git_feed

Initial (root) commit has no parents, and $co{'parent'} is
undefined. Use '--root' for initial commit.

This fixes "Use of uninitialized value in open at gitweb/gitweb.perl
line 4925." warning.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

gitweb/gitweb.perl

@@ -5040,7 +5040,8 @@ XML
 
 		# get list of changed files
 		open my $fd, "-|", git_cmd(), "diff-tree", '-r', @diff_opts,
-			$co{'parent'}, $co{'id'}, "--", (defined $file_name ? $file_name : ())
+			$co{'parent'} || "--root",
+			$co{'id'}, "--", (defined $file_name ? $file_name : ())
 			or next;
 		my @difftree = map { chomp; $_ } <$fd>;
 		close $fd