Follow-up from "Optimize ProjectConfig::Repository file existence check"

The following discussion from !127661 (merged) should be addressed:

• @drew started a discussion:

  Having to change this setup along with the implementation change, I'm wondering if it's over-stubbed. Granted, we're stubbing two method calls lower-level than we were, but I'm considering that we should go farther.

  Maybe stubbing raw_repository.blob_at? So we don't need to actually use the very slow create(:project, :repository), but we can move the stub to just outside the class we're testing. WDYT?