Speed up nanoc compilation by providing checksums for items.

Git has unique checksum for each item anyway.

Author: mostlyobvious

lib/nanoc/github.rb

@@ -23,7 +23,7 @@ def items
             identifier     = Nanoc::Identifier.new("/#{item[:name]}")
             metadata, data = decode(item[:content])
 
-            new_item(data, metadata, identifier)
+            new_item(data, metadata, identifier, checksum_data: item[:sha])
           end
         end
       end

test/source_test.rb

@@ -85,6 +85,7 @@ def test_item
             Some content.
           EOC
           assert_equal ({}), item.attributes
+          assert_equal "3c612584217e21ae00d532a86e0e35685131dbba", item.checksum_data
         end
       end