use document.referrer instead of history.length (#13697)

Contributed by Filou Centrinov.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@11900 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/views/journals/diff.html.erb

@@ -7,7 +7,7 @@
 
 <p>
   <%= link_to(l(:button_back), issue_path(@issue),
-              :onclick => 'if (history.length > 1) {history.back(); return false;}') %>
+              :onclick => 'if (document.referrer != "") {history.back(); return false;}') %>
 </p>
 
 <% html_title "#{@issue.tracker.name} ##{@issue.id}: #{@issue.subject}" %>