Bst track can't track when multiple sources are on the same element without ref.

Summary

BuildStream can never track sources that require refs on the previous sources if the previous sources don't have ref set.

This will prevent using bst track entirely on a previous element until we set a dummy ref.

Steps to reproduce

8ae41d53 is modifying the test that was checking for this behaviour in order to remove the corner case the test was actually testing.

What is the current bug behavior?

Source can't be traccked with CONSISTENCY.INCONSISTENT

What is the expected correct behavior?

both sources should have their ref updated

Relevant logs and/or screenshots

Possible fixes

Other relevant information

• BuildStream version affected: /milestone %BuildStream_v1.x

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