RevertAction: Prevent file revert if current version is identical

RevertAction: Prevent file revert if current version is identical

There are several reasons for this:

* The file thumbnail shown on the main file page is cached by the
  browser, as images tend to be. This often confuses users into
  thinking their revert did not work, and into attempting it again.
  Recent examples:
  * https://commons.wikimedia.org/wiki/Commons:Village_pump/Archive/2016/07#Wrong_SVG_rendering_on_File:DuckDuckGo_logo_and_wordmark.svg
  * https://commons.wikimedia.org/w/index.php?title=Commons:Upload_help&oldid=205348523#Reversion_is_not_working_for_me.
  Ideally we'd prevent the caching, but preventing repeated reverts
  should also work.

* Refreshing the success page causes the revert to be attempted again
  (T53383). The usual solution to this is the Post/Redirect/Get
  pattern, but we want to show a success message so that would require
  some more changes (something similar to the post-edit notification).

* It can serve as a "revert conflict" detection mechanism, crude but
  better than none.

In the unlikely case that uploading an identical older version of
the file is necessary, it can still be done using Special:Upload.

Bug: T53383
Change-Id: I37e04a536c5c2fc6cdbe59f6f598bb0c7f25d7a7