'cd' without arguments now changes to $HOME.

I had mistakenly thought that util.GetHomeDir() used $HOME and then
/etc/passwd.  It only uses /etc/passwd.

Fixes issue #24.

core/builtin.py

@@ -406,9 +406,15 @@ def Cd(argv, mem):
  try:
    dest_dir = argv[0]
  except IndexError:
    # NOTE: This is equivalent to 'cd ~', but bash/mksh only seem to respect
    # $HOME, and not /etc/passwd.  OSH behavior is different but better.
    dest_dir = util.GetHomeDir()
    val = mem.GetVar('HOME')
    if val.tag == value_e.Undef:
      util.error("$HOME isn't defined")
      return 1
    elif val.tag == value_e.Str:
      dest_dir = val.s
    elif val.tag == value_e.StrArray:
      util.error("$HOME shouldn't be an array.")
      return 1

  if dest_dir == '-':
    old = mem.GetVar('OLDPWD', scope.GlobalOnly)

spec/builtins.test.sh

@@ -49,8 +49,10 @@ cd -
# stdout-json: "old: /\n/\n"

### cd with no arguments
HOME=$TMP/home
mkdir -p $HOME
cd
test $(pwd) = ~ && echo OK
test $(pwd) = "$HOME" && echo OK
# stdout: OK

### pushd/popd

test/sh_spec.py

@@ -856,10 +856,8 @@ def main(argv):
  if not opts.path_env:
    raise RuntimeError('--path-env required')
  env = {
    'TMP': opts.tmp_env,
    'TMP': os.path.normpath(opts.tmp_env),  # no .. or .
    'PATH': opts.path_env,
    # The normal case for ~ and 'cd' is that $HOME is defined.
    'HOME': os.environ['HOME'],
  }
  stats = RunCases(cases, case_predicate, shell_pairs, env, out)
  out.EndCases(stats)