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filemerge: fix backing up an in-memory file to a custom location

Authored by phillco.

Description

filemerge: fix backing up an in-memory file to a custom location

If the user specifies a ui.origbackuppath, we used to always copy the file
there, but if the source file is in memory we must write it instead of copying.

Differential Revision: https://phab.mercurial-scm.org/D1806