py3: use list comprehensions instead of filter where we need to eagerly filter
These two uses of filter() are then checked for truthiness, but on Python 3:
>>> bool(filter(None, []))
True
So we need to stop depending on that. Fortunately it's easy to replace
the filter with an equivalent list comprehension.
Differential Revision: https://phab.mercurial-scm.org/D2364
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