Mark Commit persisted

What does this MR do?

Mark commit as persisted so that polymorphic_url of Rails could correctly generate a url for namespaced project commit actions. For specific reasons in which this is needed, cf,

https://github.com/rails/rails/blob/v4.2.6/actionpack/lib/action_dispatch/routing/polymorphic_routes.rb#L298

Are there points in the code the reviewer needs to double check?

This is a global change to all Commit objects which might introduce reactions in other parts of the program.

Why was this MR needed?

As in !3514 (merged), polymorphic_url are needed to clean up the code, since more actions for namespaced project commit are coming.

What are the relevant issue numbers?

N/A

Screenshots (if relevant)