Fix a memory leak introduced in the last release

Fix a memory leak introduced in the last release

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.