scm: use nil instead of @rev at revision text box of "View all revisions" page

@rev is set as default branch at controller.
On Git, in most cases it is "master".
And it is set on revision text box of "View all revisions" page.
"master" branch is NOT revision, so clicking OK button causes error.
It is very confused.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@10713 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/views/repositories/revisions.html.erb

@@ -4,7 +4,7 @@
         :repository_id => @repository.identifier_param},
        :method => :get
      ) do %>
-  <%= l(:label_revision) %>: <%= text_field_tag 'rev', @rev, :size => 8 %>
+  <%= l(:label_revision) %>: <%= text_field_tag 'rev', nil, :size => 8 %>
   <%= submit_tag 'OK' %>
 <% end %>
 </div>