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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL