Add type hint to Title::equals(); this lets the error log give us something useful...

Add type hint to Title::equals(); this lets the error log give us something useful like this:
  Catchable fatal error: Argument 1 passed to Title::equals() must be an instance of Title, null given, called in /Library/WebServer/Documents/trunk/includes/Linker.php on line 1323 and defined in /Library/WebServer/Documents/trunk/includes/Title.php on line 3003
instead of something useless like this:
  Fatal error: Call to a member function getInterwiki() on a non-object in /Library/WebServer/Documents/trunk/includes/Title.php on line 3005

This particular error was caused by a regression in r38149, which uses $wgTitle in Linker::doEditSectionLink(); this fails when doing parses via the API at perhaps at other times.

See bug 14965 (https://bugzilla.wikimedia.org/show_bug.cgi?id=14965)