Robustify operation when working on files within directories which are symlinked from outside

[yarikoptic]

As discovered during neurosynth hackathone (but now forgotten details) files within symlinked from outside directories would be skipped during collection of "provenance" information. For example

$> ln -s /tmp /tmp/TMP                                                                                 

$> TMPDIR=/tmp/TMP nosetests -sv  testkraut/tests/test_utils.py:test_strace_wrapper         
/usr/bin/nosetests:5: UserWarning: Module dap was already imported from None, but /usr/lib/python2.7/dist-packages is being added to sys.path
  from pkg_resources import load_entry_point
testkraut.tests.test_utils.test_strace_wrapper ... FAIL

======================================================================
FAIL: testkraut.tests.test_utils.test_strace_wrapper
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/yoh/deb/gits/pkg-exppsy/testkraut/testkraut/tests/utils.py", line 25, in newfunc
    func(*((wdir,) + arg), **kwargs)
  File "/home/yoh/deb/gits/pkg-exppsy/testkraut/testkraut/tests/test_utils.py", line 48, in test_strace_wrapper
    assert_equal(exe['uses'], set(['inf']))
AssertionError: Items in the second set but not the first:
'inf'

----------------------------------------------------------------------
Ran 1 test in 0.119s

FAILED (failures=1)