tests/test-gendoc-pt_BR.t

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

#require docutils gettext

  $ $TESTDIR/check-gendoc pt_BR
  checking for parse errors