Move showDiff from target level to annotation level.

Calculate the `showDiff` flag for the whole annotation,
instead of on a per-target basis.

Benefits:
 - One less variable stored inside the annotation model
 - We can more easily avoid unwanted overrides of the manually set value.

h/static/scripts/directives/annotation.coffee

@@ -205,9 +205,9 @@ AnnotationController = [
       # Form the tags for ngTagsInput.
       @annotation.tags = ({text} for text in (model.tags or []))
 
-      # Calculate the visual diff flag
-      @annotation.target?.forEach (t) ->
-        t.showDiff ?= t.diffHTML? and not t.diffCaseOnly
+      # Calculate the visual diff flags
+      @hasDiff = @annotation.target.filter((t) -> t.diffHTML?).length > 0
+      @showDiff ?= @hasDiff && @annotation.target.filter((t) -> t.diffHTML? and not t.diffCaseOnly).length > 0
 
     # Export the baseURI for the share link
     this.baseURI = documentHelpers.baseURI

h/templates/client/annotation.html

@@ -44,22 +44,22 @@
 <section class="annotation-target"
          ng-repeat="target in vm.annotation.target track by $index">
   <blockquote class="annotation-quote"
-              ng-hide="target.showDiff"
+              ng-hide="vm.showDiff"
               ng-bind-html="selector.exact"
               ng-repeat="selector in target.selector
                          | filter : {'type': 'TextQuoteSelector'}
                          track by $index"></blockquote>
   <blockquote class="annotation-quote"
 	      ng-bind-html="target.diffHTML"
-	      ng-show="target.showDiff"></blockquote>
-  <div class="small pull-right"
-       ng-show="target.diffHTML">
+	      ng-show="vm.showDiff"></blockquote>
+</section>
+<div class="small pull-right"
+     ng-show="vm.hasDiff">
   <input type="checkbox"
-	   ng-model="target.showDiff"
+         ng-model="vm.showDiff"
          ng-click="$event.stopPropagation()"> Show differences</input>
-  </div>
+</div>
 
-</section>
 <!-- / Excerpts -- >
 
 <!-- Body -->