[RFC] FIX JENKINS-18574/JENKINS-16045

This passes all tests, but besides limited testing with some externals, I haven't tried this. In fact, I have no idea how to test whether this actually resolves the "Maven incremental build" issue — I just made sure that `AffectedFile.getPath()` adheres to the contract as I understand it.

So please provide feedback.

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This is in parts similar to @ndeloof's ec606ac7557772cb07e72f00c084203810ebd815, but instead of rewriting the path when creating the changelog, a new attribute with the path relative to the workspace root is stored, and returned in `SubversionChangeLogSet.Path.getPath()` to fit the `AffectedFile` contract. The existing path relative to repository root can be accessed as `SubversionChangeLogSet.Path.getValue()`, so existing consumers like repo browsers only need to switch to that method, if they're not already using it. In fact, all bundled repo browsers already use `getValue()`, likely because it [predates](https://github.com/jenkinsci/jenkins/commit/3f4f8292e93f16bd5512e655ed860de623e40f32) `getPath()`.