tests/test-username-newline.t

windows: add a global equivalent to /etc/mercurial for *.rc processing

This follows the Unix model of processing this directory immediately after
<internal>/*.rc, and prior to the installation relative files.  Since the Unix
processing supports both a directory and a file (the former overriding the
latter), and since %HOME% supports both `*.ini` and `.hgrc` (again, the former
overriding the latter), this does too.  The Unix file doesn't have a `.` prefix,
so it's not used here either.

Note that this is the opposite order of processing the exe relative paths.  But
since it's in agreement with Unix, %HOME% and %USERPROFILE%, it seems reasonable
to ignore that.  Maybe we can change that and take a BC, because that's
something the installer should be controlling, and I can't imagine people having
both paths *and* conflicting settings.

Differential Revision: https://phab.mercurial-scm.org/D7692

  $ hg init
  $ touch a

  $ unset HGUSER
  $ echo "[ui]" >> .hg/hgrc
  $ echo "username= foo" >> .hg/hgrc
  $ echo "          bar1" >> .hg/hgrc

  $ hg ci -Am m
  adding a
  abort: username 'foo\nbar1' contains a newline
  
  [255]
  $ rm .hg/hgrc

  $ HGUSER=`(echo foo; echo bar2)` hg ci -Am m
  adding a
  abort: username 'foo\nbar2' contains a newline
  
  [255]
  $ hg ci -Am m -u "`(echo foo; echo bar3)`"
  adding a
  transaction abort!
  rollback completed
  abort: username 'foo\nbar3' contains a newline!
  [255]