New hook MessageCache::get

New hook MessageCache::get

Example usage:

$wgHooks['MessageCache::get'][] = function( &$key ) {
	static $keys = null;
	if ( $keys === null ) {
		global $wgExtensionMessagesFiles;
		require( $wgExtensionMessagesFiles['OverrideMessages'] );
		$keys = array_flip( array_keys( $messages['en'] ) );
	}

	if ( isset( $keys["myprefix-$key"] ) ) {
		$key = "myprefix-$key";
	}

	return true;
}

Pros:
* Easy way to override standard core and extension messages without
  any changes to them
* Messages can be stored in a standard i18n file
* Messages can be translated easily with Translate
* Messages can be shared accross multiple wikis easily
* Takes advantage of the normal message cache behavior unlike the
  MessagePreLoad hook
* Missing translations fallback to the override, not to the
  uncustomized standard translation
* Do not need to handle conflicting message keys at translatewiki.net
  if adopted by WMF

Cons:
* This method is called often, so there will be small performance
  impact if no hooks are registered. Impact can be big if the
  implementation of hook subscriber is inefficient.

This can help with bugs like 36149. It doesn't remove the manual work
needed to detect those messages and adding them to the i18n file.

I have been using this patch in a wiki farm for months.

Change-Id: Ib39937a440e71ae7292cf992ab37a569189741e4