The long story: I want to call this function from RefsHelper; however, I can't
make WorkingTreeHelper a field of RefsHelper because RefsHelper is already a
field in WorkingTreeHelper, so that would be a circular dependency.
The shorter story: there's really little reason to have to instantiate a helper
object in order to call a simple function like this. Long term I would like to
get to a state where a lot more of these helper functions are free-standing, and
you pass in the data they need.
While at it, simplify the implementation of AnyStagedFiles and AnyTrackedFiles
to one-liners.
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