Fix #339055 and #277444 by handling non ascii symlink targets.

* bzrlib/tests/tree_implementations/test_path_content_summary.py:
(TestPathContentSummary.test_unicode_symlink_target_summary): Test
adding a non ascii symlink target.

* bzrlib/tests/workingtree_implementations/test_parents.py:
(TestSetParents.test_unicode_symlink): Should not fail anymore

* bzrlib/tests/branch_implementations/test_sprout.py:
(TestSprout.test_sprout_with_unicode_symlink): Should not fail anymore.

* bzrlib/workingtree.py:
(WorkingTree.path_content_summary): Get the symlink target as
unicode.

* bzrlib/transform.py:
(_PreviewTree.path_content_summary): Get the symlink target as
unicode.

* bzrlib/dirstate.py:
(DirState._read_link): abspath should stay in Unicode.
(DirState._inv_entry_to_details): Encode path in utf8 for the
fingerprint.