gitweb: esc_html() site name for title in OPML

This escapes the site name in OPML (XML uses the same escaping rules
as HTML).  Also fixes encoding issues because esc_html() uses
to_utf8().

Signed-off-by: Jürgen Kreileder <jk@blackdown.de>
Acked-by: Jakub Narębski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/gitweb/gitweb.perl b/gitweb/gitweb.perl
index b077032..a9f39d5 100755
--- a/gitweb/gitweb.perl
+++ b/gitweb/gitweb.perl
@@ -7700,11 +7700,12 @@
 		-charset => 'utf-8',
 		-content_disposition => 'inline; filename="opml.xml"');
 
+	my $title = esc_html($site_name);
 	print <<XML;
 <?xml version="1.0" encoding="utf-8"?>
 <opml version="1.0">
 <head>
-  <title>$site_name OPML Export</title>
+  <title>$title OPML Export</title>
 </head>
 <body>
 <outline text="git RSS feeds">