diff: replace --merge option by config option I can't think of any reason you'd want to enable the merge diff on a run-to-run basis; you'd probably either always or never want it set (though I can't see why you'd never want it set). If you have it set, you'll probably also want the same output in `hg log -p` output. Having a single config option for the feature makes sense. Differential Revision: https://phab.mercurial-scm.org/D9956

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