filemerge: when using in-memory merge, always put backup files in temp dir Before calling a merge tool, we create a backup of the local side of the merge. That file can be put in the working copy or in a temporary directory, depending on the user's config. When we're merging in memory, we don't want to write to the actual, on-disk working copy, so we write the file to the in-memory working copy instead. However, since we don't support external merge tools with in-memory merge, it makes no difference where the file is actually stored (and if we ever do add support for external merge tools, then the file clearly can't live in the in-memory working-copy object anyway). So, since it doesn't matter where the file is stored, we can simplify by always putting them in the system's temp directory. Differential Revision: https://phab.mercurial-scm.org/D12187

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