tests/test-username-newline.t

upgrade: Use `improvement` subclasses everywhere, not instances

This changes the source definition of optimizations to match that of formats:
a subclass with a decorator, instead of an instance passed to a function call.
Not having any instance removes the confusion between class attributes and
instance attributes, which were used interchangeably.

Differential Revision: https://phab.mercurial-scm.org/D10768

  $ 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
  [50]