Breakage due to CallStack refactoring

[Ben Gamari]

Hi Simon,

In 317236db308d you performed some refactoring of CallStack defaulting
and noted that there was no change in visisble behavior. However, it
isn't seem that this is true. The following two tests [1] now fail as
callstacks are no longer produced in their output,

    TEST="assert T10845"

Presumably this isn't expected.

Cheers,

- Ben


[1] https://phabricator.haskell.org/harbormaster/build/14964/?l=100
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