merge-tools: allow marking a mergetool as completely disabled Very often in my life I'm finding that the only configured merge tool present on the system is vimdiff[0], and it's currently impossible (as far as I can tell) short of specifying `ui.merge = `[1] to actually *disable* a merge tool. This allows vimdiff-haters to put: [merge-tools] vimdiff.disable = yes in their ~/.hgrc and never see vimdiff again. I'm stopping short of putting this as a commented out entry in the sample new user hgrc (seen when a user runs `hg config --edit` with no ~/.hgrc) for now, but I might come back and do that later. 0: vimdiff is at an awkward intersection: it's usually installed by the vim package which is often installed as a vi substitute, so it's mere presence doesn't imply me wanting it, unlike (say) kdiff3. 1: There's a related problem I ran into today where specifying `ui.merge = :merge` failed because :merge isn't a command, which I think is a regression. I'll try and figure that out and at least file a bug.