The problem was this line:
cleanup = and <$> sequence (map snd v)
That caused all of v to be held onto until the end, when the cleanup action
was run.
I could not seem to find a bang pattern that avoided the leak, so I
resorted to a IORef, rather clunky, but not a performance problem because
it will only be written once per git ls-files, so typically just 1 time.
This commit was sponsored by Mark Reidenbach on Patreon.
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