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

mergestate: initialize all properties in __init__() This is hopefully not very controverial. I found the initialization before this patch unorthodox. It wasn't clear which properties the object was supposed to have. Differential Revision: https://phab.mercurial-scm.org/D9037

mergestate: remove unnecessary clearing of `localctx` and `otherctx` As noted in the previous commit, there are no callers that (re-)use the instance after calling `reset()`. There are also no callers that call `_read()` after doing anything with the instance (it's only called right after an instance is created). If reviewers feel that this is too risky, I can extract and reuse the poisoning code that indygreg once added for poisining repo instances. Differential Revision: https://phab.mercurial-scm.org/D9036