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filemerge: use fctx.write() in the internal:dump tool, instead of copy

Authored by phillco.

Description

filemerge: use fctx.write() in the internal:dump tool, instead of copy

This is slower but allows this tool to work with the "deferred writes"
milestone of in-memory merge.

The performance hit is not too noticiable since this only used for the :dump
merge tool during a conflict.

Differential Revision: https://phab.mercurial-scm.org/D617