bdiff: add a xdiffblocks method This is similar to `bdiff.blocks`, but uses xdiff as the backend. The indent heuristic is turned on by default since it has little overhead and improves diff quality significantly. Differential Revision: https://phab.mercurial-scm.org/D2602

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    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}