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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