(bug 37209) Title no longer relies on entries already being in LinkCache

This fixes "LinkCache doesn't currently know about this title" error
in some cases, for example, in the case when LinkCache is flushed.

Change-Id: I2fc3966ee5d462ba050a6c54cfda1a0dfd972635

RELEASE-NOTES-1.21

@@ -140,6 +140,7 @@ production.
 * (bug 41733) Hide "New user message" (.usermessage) element from printable view.
 * (bug 39062) Special:Contributions will display changes that don't have
   a parent id instead of just an empty bullet item.
+* (bug 37209) "LinkCache doesn't currently know about this title" error fixed.
 * wfMerge() now works if $wgDiff3 contains spaces
 
 === API changes in 1.21 ===

includes/Title.php

@@ -2986,6 +2986,7 @@ class Title {
 			return $this->mLatestID = 0;
 		}
 		$linkCache = LinkCache::singleton();
+		$linkCache->addLinkObj( $this );
 		$cached = $linkCache->getGoodLinkFieldObj( $this, 'revision' );
 		if ( $cached === null ) { # check the assumption that the cache actually knows about this title
 			# XXX: this does apparently happen, see https://bugzilla.wikimedia.org/show_bug.cgi?id=37209