Revisions: simpler hash URLs. Misc refactoring.

* Single mode: `#at/:to`
* Compare two mode: `#from/:from/to/:to`
* Make use of `_.isUndefined()`

See #24425.

git-svn-id: http://core.svn.wordpress.org/trunk@24638 1a063a9b-81f0-0310-95a4-ce76da25c4cd