add unit test of Redmine::Helpers::Diff (#15105)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12205 e93f8b46-1217-0410-a6f0-8f06a7374b81
add missing "require 'diff'" to Redmine::Helpers::Diff (#15105)

Contributed by Felix Bünemann.

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12206 e93f8b46-1217-0410-a6f0-8f06a7374b81
remove unneeded "require 'diff'" from WikiController (#15105)

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@12207 e93f8b46-1217-0410-a6f0-8f06a7374b81