tests/test-username-newline.t

sparse: move code for importing rules from files into core

This is a pretty straightforward port. Some code cleanup was
performed. But no major changes to the logic were made.

I'm not a huge fan of this function because it does multiple
things. I'd like to get things into core first to facilitate
refactoring later.

Please also note the added inline comment about the oddities
of writeconfig() and the try..except to undo it. This is because
of the hackiness in which the sparse matcher is obtained by
various consumers, notably dirstate. We'll need a massive
refactor to address this. That refactor is effectively blocked
on having the sparse dirstate hacks live in core.

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