repo: use reverse post-order DFS in repro --downstream

[pmrowla]

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Will close #3602.

from original issue:

Looks like the issue is that for --downstream, we are reversing the DAG, and then traversing the reversed graph w/DFS pre-ordered search. In the example case, using pre-ordered search provides no guarantee that step3 is only visited after step2.
...
I think what we should be doing is traversing the reversed graph w/reverse post-ordered search, to ensure that dependencies are run in the correct order.

[pmrowla]

test_pull_git_imports is occasionally failing on travis, but unrelated to this issue (see #3570)