$ hg init

  $ echo a > a

  $ hg ci -qAm 'add a'

  $ hg init subrepo

  $ echo 'subrepo = http://example.net/libfoo' > .hgsub

  $ hg ci -qAm 'added subrepo'

  $ hg up -qC 0

  $ echo ax > a

  $ hg ci -m 'changed a'

  created new head

  $ hg up -qC 1

  $ cd subrepo

  $ echo b > b

  $ hg add b

  $ cd ..

Should fail, since there are added files to subrepo:

  $ hg merge

  abort: uncommitted changes in subrepository 'subrepo'

  [255]