Revisions: real URLs and preloading of the requested diff.

* Real URLs are being used now, using pushState. `?revision={id}` or `?from={from}&to={to}`.
* Drop the redundant `action=edit` from the URLs (this is the default).
* The initial comparison is preloaded, whether a single revision or a compare-two situation.

See #24425.

git-svn-id: http://core.svn.wordpress.org/trunk@24664 1a063a9b-81f0-0310-95a4-ce76da25c4cd