Fallback to read from master in DifferenceEngine::loadRevisionData

In DifferenceEngine we need to show a diff immediately after
an edit happened if someone did a rollback. To make that work
again, try loading a revision from master if we have more than
one database servers and we failed loading the revision from
a slave.

Change-Id: I08a16faa9e637c614c6d2fc9a10536c7b41685a8
Follows: 3c2bc32ae1
Bug: T93866

includes/diff/DifferenceEngine.php

@@ -1233,6 +1233,12 @@ class DifferenceEngine extends ContextSource {
 		// Load the new revision object
 		if ( $this->mNewid ) {
 			$this->mNewRev = Revision::newFromId( $this->mNewid );
+
+			if ( !$this->mNewRev && wfGetLB()->getServerCount() > 1 ) {
+				// Try harder… This is being hit after a rollback where we show the
+				// diff immediately after the edit happened. T93866
+				$this->mNewRev = Revision::newFromId( $this->mNewid, Revision::READ_LATEST );
+			}
 		} else {
 			$this->mNewRev = Revision::newFromTitle(
 				$this->getTitle(),