rhg: faster hg cat when many files are requested With this patch I'm seeing a ~39ms improvement (220ms -> 181ms) when running [hg cat] on ~220 files in a ~260k-file repo. The timing for [hg cat] on an individual file becomes slightly worse (losing 5ms: 145ms -> 150ms). A follow-up commit is intended to improve that. Differential Revision: https://phab.mercurial-scm.org/D11615

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