Customizer: Don't trigger a change event if two unchanged object values are equal, second pass.

Make Underscore.js a dependency for `customize-base` and use `_.isEqual()` to compare the values.
(Underscore.js was already enqueued via wp-util.js for Widgets.)

props westonruter.
fixes #26061.
Built from https://develop.svn.wordpress.org/trunk@29907


git-svn-id: http://core.svn.wordpress.org/trunk@29661 1a063a9b-81f0-0310-95a4-ce76da25c4cd