add new "label_diff" at en.yml for hardcoded string "diff" in Repositories_Helper and Wiki#show (#9043).

Contributed by Tom Rochette.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@6421 e93f8b46-1217-0410-a6f0-8f06a7374b81

config/locales/en.yml

@@ -694,6 +694,7 @@ en:
   label_statistics: Statistics
   label_commits_per_month: Commits per month
   label_commits_per_author: Commits per author
+  label_diff: diff
   label_view_diff: View differences
   label_diff_inline: inline
   label_diff_side_by_side: side by side