exchange: move disabling of rev-branch-cache bundle part out of narrow I'm attempting to refactor changegroup code in order to better support alternate storage backends. The narrow extension is performing a lot of monkeypatching to this code and it is making it difficult to reason about how everything works. I'm reasonably certain I would be unable to abstract storage without requiring extensive rework of narrow. I believe it is less effort to move narrow code into core so it can be accounted for when changegroup code is refactored. So I'll be doing that. The first part of this is integrating the disabling of the cache:rev-branch-cache bundle2 part into core. This doesn't seem like it is related to changegroup, but narrow's modifications to changegroup are invasive and also require taking its code for bundle generation and exchange into core in order for the changegroup code to work. Differential Revision: https://phab.mercurial-scm.org/D4007

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