$ 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

  $ rm .hg/hgrc

  $ HGUSER=`(echo foo; echo bar2)` hg ci -Am m

  abort: username 'foo\nbar2' contains a newline

  $ hg ci -Am m -u "`(echo foo; echo bar3)`"

  transaction abort!

  rollback completed

  abort: username 'foo\nbar3' contains a newline!

  $ true