tests/test-username-newline.t

parsers: clarify documentation of test-parseindex2.py

This change updates and improves the description of test-parseindex2.py.
In particular, it removes language that can be interpreted to mean that the
test module checks only the C implementation of parsers.parse_index2().
Rather, the module checks parsers.parse_index2(), which can be either the
C or pure Python implementation, depending on which version is being used.

As of e57c532c3835, the module also does more than just compare the return
value with the original Python implementation.

  $ 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]