fetchpatch: add `relative`

Allows restricting patches to a specific subdirectory, à la
`git diff --relative=subdir`.

This cannot be done (cleanly) currently because the `includes` logic
happens *after* `stripLen` is applied, so we can't match on `subdir/*`.

This change adds a `relative` argument that makes this possible by
filtering files before doing any processing, and setting `stripLen` and
`extraPrefix` accordingly.

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