DRY up the specs for Projects::CompareController

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The following discussion from !51985 (merged) should be addressed:

• @10io started a discussion: (+1 comment)

  Food for thoughts:

  All these contexts seem to play with different projects/forks and different visibilities. This could be summarized in a table based spec to have a better view of what is allowed and what is not allowed.

  This is certainly out of the scope of this MR.

/cc @nick.thomas