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Some others IDE (like PhpStorm) have feature that allows you to select piece of code and compare it with content of clipboard and display it the same way as if you compare 2 versions of versioned document (see PhpStorm help http://www.jetbrains.com/phpstorm/webhelp/comparing-files.html#clipboard)
Not related to version control. To diff a clipboard content with a piece of editor code, use the diff API to display the diff view. Reassigning to editor because the view probably needs to be displayed from there.
There is a third party plugin, which inplements this RFE. http://plugins.netbeans.org/plugin/31189/compare-with-clipboard