exchange: move narrow acl functionality into core This function is called by the custom changegroup generation code in the narrow extension. I want to move that changegroup code into core. That means we need to move this function. The code is kinda hacky in that assumes existence of REMOTE_USER, which is only present on authenticated HTTP requests. I've added a comment indicating that. Differential Revision: https://phab.mercurial-scm.org/D4008

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