Add transition.transition.

This derives a new transition from the existing transition, equivalent to (and
implemented as) transition.select(function() { return this; }).

Makefile

@@ -40,6 +40,7 @@ d3.core.js: \
 	src/core/date.js \
 	src/core/object.js \
 	src/core/array.js \
+	src/core/this.js \
 	src/core/functor.js \
 	src/core/rebind.js \
 	src/core/ascending.js \
@@ -113,6 +114,7 @@ d3.core.js: \
 	src/core/transition-delay.js \
 	src/core/transition-duration.js \
 	src/core/transition-each.js \
+	src/core/transition-transition.js \
 	src/core/timer.js \
 	src/core/noop.js
 

d3.js

@@ -18,6 +18,9 @@ function(array, prototype) {
 function(array, prototype) {
   for (var property in prototype) array[property] = prototype[property];
 };
+function d3_this() {
+  return this;
+}
 d3.functor = function(v) {
   return typeof v === "function" ? v : function() { return v; };
 };
@@ -1949,6 +1952,9 @@ function d3_transition_each(callback) {
   }
   return this;
 }
+d3_transitionPrototype.transition = function() {
+  return this.select(d3_this);
+};
 var d3_timer_queue = null,
     d3_timer_interval, // is an interval (or frame) active?
     d3_timer_timeout; // is a timeout active?

src/core/this.js

@@ -0,0 +1,3 @@
+function d3_this() {
+  return this;
+}

src/core/transition-transition.js

@@ -0,0 +1,3 @@
+d3_transitionPrototype.transition = function() {
+  return this.select(d3_this);
+};