Replace different_project? method with instance variable

What does this MR do and why?

While reading the code to solve #424579 I found that different_project? method can only be called after create_commit method, which sets necessary instance variables being checked. The MR removes that level of indirection by letting create_commit set @different_project field explicitly.

This will also help to replace concern with composition in future (#16270), where create_commit could return an error object with all necessary fields to render, instead of injecting the fields into including class.

How to set up and validate locally

#424579

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