Lazily load LiteBlob#data

Some operations, notably `Repository#delete_blob`, don't need access to the
blob content. Loading it from disc in these cases is unnecessary.

This commit lazily loads the content on first access and memoizes it. If it
is never accessed, it will never be loaded.

This should reduce disc access significantly for commits that delete lots of
files.

lib/elasticsearch/git/lite_blob.rb

@@ -7,18 +7,21 @@ module Elasticsearch
       include Linguist::BlobHelper
       include Elasticsearch::Git::EncoderHelper
-      attr_accessor :id, :name, :path, :data, :size, :mode, :commit_id
+      attr_accessor :id, :name, :path, :size, :mode, :commit_id
+      attr_writer :data
       def initialize(repo, raw_blob_hash)
         @id   = raw_blob_hash[:oid]
-
-        blob  = repo.lookup(@id)
+        @blob  = repo.lookup(@id)
         @mode = raw_blob_hash[:mode].to_s(8)
-        @size = blob.size
+        @size = @blob.size
         @path = encode!(raw_blob_hash[:path])
         @name = @path.split('/').last
-        @data = encode!(blob.content)
+      end
+
+      def data
+        @data ||= encode!(@blob.content)
       end
     end
   end