rename .rhtml to .html.erb of app/views/common/_diff.rhtml.

:rhtml and :rxml were finally removed as template handlers at Rails 3.1 RC4.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@6548 e93f8b46-1217-0410-a6f0-8f06a7374b81