Core: allow init to accept an alternate rootjQuery for migrate's sake

- This will be tested in migrate

Fixes gh-2101

src/core/init.js

@@ -15,7 +15,7 @@ var rootjQuery,
 	// Shortcut simple #id case for speed
 	rquickExpr = /^(?:\s*(<[\w\W]+>)[^>]*|#([\w-]+))$/,
 
-	init = jQuery.fn.init = function( selector, context ) {
+	init = jQuery.fn.init = function( selector, context, root ) {
 		var match, elem;
 
 		// HANDLE: $(""), $(null), $(undefined), $(false)
@@ -23,6 +23,10 @@ var rootjQuery,
 			return this;
 		}
 
+		// init accepts an alternate rootjQuery
+		// so migrate can support jQuery.sub (gh-2101)
+		root = root || rootjQuery;
+
 		// Handle HTML strings
 		if ( typeof selector === "string" ) {
 			if ( selector[0] === "<" &&
@@ -81,7 +85,7 @@ var rootjQuery,
 
 			// HANDLE: $(expr, $(...))
 			} else if ( !context || context.jquery ) {
-				return ( context || rootjQuery ).find( selector );
+				return ( context || root ).find( selector );
 
 			// HANDLE: $(expr, context)
 			// (which is just equivalent to: $(context).find(expr)
@@ -98,8 +102,8 @@ var rootjQuery,
 		// HANDLE: $(function)
 		// Shortcut for document ready
 		} else if ( jQuery.isFunction( selector ) ) {
-			return rootjQuery.ready !== undefined ?
-				rootjQuery.ready( selector ) :
+			return root.ready !== undefined ?
+				root.ready( selector ) :
 				// Execute immediately if ready is not present
 				selector( jQuery );
 		}