gitweb: No difftree output for trivial merge

In 'commitdiff' view, for the merge commit, there is an extra header
for the difftree table, with links to commitdiffs to individual
parents.  Do not show such header when there is nothing to show, for
trivial merges.

This means that for trivial merge you have to go to 'commit' view
to get links to diffs to each parent.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>

gitweb/gitweb.perl

@@ -2809,7 +2809,7 @@ sub git_difftree_body {
 	      "diff_tree\">\n";
 
 	# header only for combined diff in 'commitdiff' view
-	my $has_header = @parents > 1 && $action eq 'commitdiff';
+	my $has_header = @$difftree && @parents > 1 && $action eq 'commitdiff';
 	if ($has_header) {
 		# table header
 		print "<thead><tr>\n" .