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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