If a file is overridden and then overridden again, this caused us to
rebuild only the first thing that depends on it, but not any subsequent
things, which is a pretty serious bug.
It turned out that t/350-deps-forget is already supposed to test this,
but I had cleverly encoded the wrong behaviour into the expected
results in the table-driven test. I blame lack of sleep. Anyway, I
fixed the test, which made it fail, and then fixed the code, which made
it pass.
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