Add `respond_to_missing?` for Commit and Repository

As of Ruby 1.9, this is the correct way to handle `respond_to?` for
methods implemented by `method_missing`.

See https://robots.thoughtbot.com/always-define-respond-to-missing-when-overriding

app/models/commit.rb

@@ -172,10 +172,8 @@ class Commit
     @raw.send(m, *args, &block)
   end
 
-  def respond_to?(method)
-    return true if @raw.respond_to?(method)
-
-    super
+  def respond_to_missing?(method, include_private = false)
+    @raw.respond_to?(method, include_private) || super
   end
 
   # Truncate sha to 8 characters

app/models/repository.rb

@@ -163,10 +163,8 @@ class Repository
     end
   end
 
-  def respond_to?(method)
-    return true if raw_repository.respond_to?(method)
-
-    super
+  def respond_to_missing?(method, include_private = false)
+    raw_repository.respond_to?(method, include_private) || super
   end
 
   def blob_at(sha, path)