Set $HOME up front, like other environment variables.

If it wasn't already set, it would be evaluated on every ~.

core/state.py

@@ -417,6 +417,11 @@ def _InitVarsFromEnv(self, environ):
        ast.LhsName('SHELLOPTS'), None, (var_flags_e.ReadOnly,),
        scope_e.GlobalOnly)

    v = self.GetVar('HOME')
    if v.tag == value_e.Undef:
      home_dir = util.GetHomeDir() or '~'  # No expansion if not found?
      SetGlobalString(self, 'HOME', home_dir)

  def SetSourceLocation(self, source_name, line_num):
    # Mutate Str() objects.
    self.source_name.s = source_name

core/word_eval.py

@@ -176,16 +176,10 @@ def _EvalTildeSub(self, prefix):
    if prefix == '':
      # First look up the HOME var, and then env var
      val = self.mem.GetVar('HOME')
      if val.tag == value_e.Str:
        return val.s
      elif val.tag == value_e.StrArray:
        raise AssertionError

      s = util.GetHomeDir()
      if s is None:
        s = '~' + prefix  # No expansion I guess
      return s
      assert val.tag == value_e.Str, val
      return val.s

    # For ~otheruser/src.  TODO: Should this be cached?
    # http://linux.die.net/man/3/getpwnam
    try:
      e = pwd.getpwnam(prefix)