[changed-files] return absolute platform paths

[mikeharder]

First attempt. I don't like adding the options bag to the filters, it also makes them always require a lambda to use.

To make the filters most reusable, I thinke we need to decompose them, to their minimal parts, so callers can decide if they need to combine checks with startsWithSpecification(repoRoot).

It would also be better, if we could filter the call to getChangedFiles() to the spec folder initially, when it makes sense. This way, maybe the filters never need to check for "starts with spec" vs "spec folder nested under test dir".

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