Use containsPath where possible

[robertbrignull]

Following on from #2509, these are two places that were doing their own check of if a path contains another path. We can use containsPath instead and it shouldn't change any behaviour, or it might make it more correct.

I haven't looked too hard into how this code is used, but only at the low level code. It should be equivalent, but I can run some tests if needed.

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