We don't need to maintain additional state to allow this; all we need to do is
take over the filter only when the number of conflicting files goes from zero to
non-zero, rather than every time it is non-zero.
The only problem is that we don't allow users to go back to showing only
conflicted files, but that's just because we don't have that as an entry in the
menu. And I don't think it's a problem.
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