tests/test-merge-subrepos.t

dirstate.status: don't ignore symlink placeholders in the normal set

On Windows, there are two ways symlinks can manifest themselves:
1. As placeholders: text files containing the symlink's target. This is what
   usually happens with fresh clones on Windows.
2. With their dereferenced contents. This happens with clones accessed over NFS
   or Samba.

In order to handle case 2, ca6cebd8734e made dirstate.status ignore all symlink
placeholders on Windows. It doesn't ignore symlinks in the lookup set, though,
since those don't have the link bit set. This is problematic because it
violates the invariant that `hg status` with every file in the normal set
produces the same output as `hg status` with every file in the lookup set.

With this change, symlink placeholders in the normal set are no longer ignored.
We instead rely on code in localrepo.status that uses heuristics to look for
suspect placeholders.

An upcoming patch will test this out by no longer adding files written in the
last second of an update to the lookup set.

  $ 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]