Cache include file to save liquid parsing time.

features/include_tag.feature

@@ -77,3 +77,15 @@ Feature: Include tags
     When I run jekyll build
     Then the _site directory should exist
     And I should see "one included" in "_site/index.html"
+
+  Scenario: Include a file and rebuild when include content is changed
+    Given I have an _includes directory
+    And I have an "_includes/one.html" file that contains "include"
+    And I have an "index.html" page that contains "{% include one.html %}"
+    When I run jekyll build
+    Then the _site directory should exist
+    And I should see "include" in "_site/index.html"
+    When I wait 1 second
+    Then I have an "_includes/one.html" file that contains "include content changed"
+    When I run jekyll build
+    Then I should see "include content changed" in "_site/index.html"

lib/jekyll/tags/include.rb

@@ -123,7 +123,7 @@ def render(context)
         end
 
         begin
-          partial = site.liquid_renderer.file(path).parse(read_file(path, context))
+          partial = load_cached_partial(path, context)
 
           context.stack do
             context['include'] = parse_params(context) if @params
@@ -134,6 +134,19 @@ def render(context)
         end
       end
 
+      def load_cached_partial(path, context)
+        cached_partials = context.registers[:cached_partials] || {}
+
+        if cached = cached_partials[path]
+          return cached
+        end
+
+        partial = context.registers[:site].liquid_renderer.file(path).parse(read_file(path, context))
+        cached_partials[path] = partial
+        context.registers[:cached_partials] = cached_partials
+        partial
+      end
+
       def resolved_includes_dir(context)
         context.registers[:site].in_source_dir(@includes_dir)
       end