storage: introduce a `revlog.reuse-external-delta-parent` config As pointed in c59987ab96b4, we had no simple way to get a client to not blindly reuse the delta parent from a bundle. Instead one had to rely on a side effect of the deprecated `format.generaldelta` configuration. We introduce an explicit `revlog.reuse-external-delta-parent` configuration option (default to True) to control this behavior. If the option is not set, `format.generaldelta` still control this behavior. To test the new option, we convert a couple of place where `generaldelta` have been used for its side effects.

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