Add CommitDecorator#short_id

app/decorators/commit_decorator.rb

 class CommitDecorator < ApplicationDecorator
   decorates :commit
+  def short_id(length = 10)
+    id.to_s[0..length]
+  end
+
   # Returns the commits title.
   #
   # Usually, the commit title is the first line of the commit message.