Avoid stack overflow in cachekeycomputer

I managed to trigger a stack overflow in Morph, by having two chunks with
the same name and same build instructions in different strata (by mistake,
obviously).

The error before was this:

    ....
      File "/src/morph/morphlib/cachekeycomputer.py", line 77, in get_cache_id
        cacheid = self._calculate(source)
      File "/src/morph/morphlib/cachekeycomputer.py", line 86, in _calculate
        for a in source.dependencies],
      File "/src/morph/morphlib/cachekeycomputer.py", line 38, in compute_key
        ret = self._hash_id(self.get_cache_id(source))
      File "/src/morph/morphlib/cachekeycomputer.py", line 77, in get_cache_id
        cacheid = self._calculate(source)
      File "/src/morph/morphlib/cachekeycomputer.py", line 83, in _calculate
        'env': self._filterenv(self._build_env.env),
      File "/src/morph/morphlib/cachekeycomputer.py", line 32, in _filterenv
        return dict([(k, env[k]) for k in keys])
    RuntimeError: maximum recursion depth exceeded while calling a Python object

The error afterwards is this:

    2016-02-18 13:53:14 Deciding on task order
    ERROR: There are multiple versions of component 'cython'

Change-Id: I96a41cabe4d9aa81bddc5186c5248a636a0843c2

0 files changed, 0 insertions, 0 deletions