filemerge: put temporary files in single temp dir by default The feature introduced in D2888 seems like a pure improvement to me. It makes the names' of temporary file easier to read. Let's have it always enabled. I also removed the config option for the path prefix because it doesn't seem useful. I asked Kyle (the author of the feature) about it and he couldn't think of a reason to keep it. I suspect it was just that we to have a config to turn it on/off while it was experimental, so it might as well be a configurable prefix then. Differential Revision: https://phab.mercurial-scm.org/D12171

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