filemerge: add configs to disable some or all partial merge tools When rolling out partial merge tools to users, it's useful to be able to easily turn one or all of them off if a problem is discovered. This patch adds support for that. They can of course also be useful for individual users to be able to temporarily turn off a tool they are otherwise using. Differential Revision: https://phab.mercurial-scm.org/D12588

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