BUG: pruning when output given unjustly drops ancestors

[ankostis]

The following graph with given-1 & given-2 inputs fails to compute with KeyError: 'a' when the asked output is asked, but it does so when not asked!

Code to reproduce:

def test_pruning_with_given_intermediate_and_asked_out():
    # Test that operations with partial inputs are pruned and not failing.
    graph = compose(name="graph")(
        operation(name="unjustly pruned", needs=["given-1"], provides=["a"])(lambda a: a),
        operat`ion(name="shortcuted", needs=["a", "b"], provides=["given-2"])(add),
        operation(name="good_op", needs=["a", "given-2"], provides=["asked"])(add),
    )

    assert graph({"given-1": 5, "b": 2, "given-2": 2}) == {"given-1": 5, "b": 2, "given-2": 7, "a": 5, "b": 2, "asked": 12}  # that ok # FAILS!
    assert graph({"given-1": 5, "b": 2, "given-2": 2}, ["asked"]) == {"asked": 12}  # FAILS!

Root cause:

• The culling logic in v1.2.4 when specific outputs are asked is to drop indiscriminately all predecessors of the given inputs, which might include values that needs to be computed.
• The existing "pruning" TCs were too simple to catch this..