tests/test-merge-subrepos.t

scmutil: split platform-specific bits into their own modules

This parallels what's done for the util module, which imports either
mercurial.posix or mercurial.windows as 'platform' and then slurps the
appropriate functions into its own namespace.

  $ 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: outstanding uncommitted changes in subrepository 'subrepo'
  [255]