Avoid unnecessary annotation renders

The pattern of comparing the new value passed to a watcher with the old
value is idiomatic angular for avoiding the first digest. Instead, the
first digest is covered by the watch on the model reference identity.
After the initial rendering, the model will not change but the other
properties may, at which point re-rendering will occur.

h/static/scripts/directives/annotation.coffee

@@ -179,6 +179,9 @@ AnnotationController = [
       # Note that copy is used so that deep properties aren't shared.
       angular.extend @annotation, angular.copy model
 
+      # Set the URI
+      @annotationURI = documentHelpers.absoluteURI("/a/#{@annotation.id}")
+
       # Extract the document metadata.
       if model.document
         uri = model.uri
@@ -221,25 +224,29 @@ AnnotationController = [
       drafts.remove model
 
     # Render on updates.
-    $scope.$watch (-> model.updated), (updated) =>
+    $scope.$watch (-> model.updated), (updated, old) =>
+      return if updated is old
       if updated then drafts.remove model
       this.render()  # XXX: TODO: don't clobber the view when collaborating
 
     # Update once logged in.
-    $scope.$watch (-> model.user), (user) =>
-      if highlight and this.isHighlight()
+    $scope.$watch (-> model.user), (user, old) =>
+      return if user is old
+      if highlight and vm.isHighlight()
         if user
           annotator.publish 'annotationCreated', model
         else
-          drafts.add model, => this.revert()
+          drafts.add model, -> this.revert()
       else
         this.render()
 
     # Calculate things neded for the visual diff support
-    $scope.$watch (-> model.target), (=> this.render()), true
+    $scope.$watch (-> model.target), (targets, old) =>
+      return if targets is old
+      this.render()
+    , true
 
-    $scope.$watch (=> @annotation.id), =>
-      vm.annotationURI = documentHelpers.absoluteURI("/a/#{@annotation.id}")
+    $scope.$watch (-> model), => this.render()
 
     # Start editing brand new annotations immediately
     unless model.id? or (highlight and this.isHighlight()) then this.edit()