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

dirstate-tracked-key: update the config value to match latest discussion Special cases are not special enough, we align the option name on the other. The `version` value is undocumented for now as it can only have a single value. It is supported in the code to properly detect and abort if more value are introduced in the future value. Differential Revision: https://phab.mercurial-scm.org/D12184

rhg: signal when falling back in logs We use the `trace` level for the actual message because it can get really busy. Differential Revision: https://phab.mercurial-scm.org/D11814