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
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).