mergestate.commit: factor out making the list of records Will aid in testing.

mergestate: move _read() call to read constructor With this patch, mergestate.clean() will no longer abort when it encounters an unsupported merge type. However we hold off on testing it until backwards compatibility is in place.

debugmergestate: explain why we create mergestate objects directly We would normally use the read() constructor, but in this case it's fine because - we implement our own reading layer, so the extra parsing done by read() is unnecessary - read() can raise an exception for unsupported merge state records, but here we'd like to handle that separately - debugmergestate needs to be privy to mergestate internals anyway