mergestate: extract overridable methods for making/restoring file backups
In-memory merge currently still uses on-disk mergestate. That's mostly
harmless (I don't think I've seen any problem it's actually
caused). It's still a little weird. I'm planning to add an in-memory
mergestate, which will be used with `overlayworkingctx`. This patch
prepares for that by extracting that logic, so it's easier to change
per subclass.
Differential Revision: https://phab.mercurial-scm.org/D9038
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