Don't link public/assets if public is linked

If the user has already added "public" to Capistrano's `:linked_dirs`,
then further linking "public/assets" is not only unnecessary, it causes
conflicts and asset precompilation fails as a result.

Fix this by checking if "public" is already linked, and if so, skip
linking "public/assets".

lib/capistrano/tasks/assets.rake

@@ -120,7 +120,11 @@ end
 # as assets_prefix will always have a default value
 namespace :deploy do
   task :set_linked_dirs do
-    set :linked_dirs, fetch(:linked_dirs, []).push("public/#{fetch(:assets_prefix)}").uniq
+    linked_dirs = fetch(:linked_dirs, [])
+    unless linked_dirs.include?('public')
+      linked_dirs << "public/#{fetch(:assets_prefix)}"
+      set :linked_dirs, linked_dirs.uniq
+    end
   end
 end