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.