Follow-up from "Allow editing with WebIDE from a commit or a tag"

The following discussion from !159557 (merged) should be addressed:

• @atevans started a discussion:

  note: (non-blocking) I feel like this is mocked out to the point where we're testing that 1 = 1, but I see it's following the pattern from the can_modify_blob specs. I'd feel more comfortable if we had an integration spec for these fields without mocking, like in the GraphQL blobs request specs