revsetbenchmark: track some simple use of "only" The only revset is quite useful and has various possible optimisation. tracking its timing seems useful.

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{

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    "conduit_uri": "https://phab.mercurial-scm.org/api",

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    "repository.callsign": "HG",

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    "arc.land.onto.default": "@",

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    "base": "hg:.^"

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}