filemerge: use arbitraryfilectx for backups
With in-memory merge, backup files might be overlayworkingfilectxs stored
in memory. But they could also be real files if the user's backup directory is
outside the working dir.
Rather than have two code paths everywhere, let's use arbitraryfilectx so they
can be consistent.
Differential Revision: https://phab.mercurial-scm.org/D1057