warnings about using syspath with some utils

Also pertaining to #58: for most utility functions, paths should *not* be
`syspath`-ified. (This only occurs right before a path is sent to the OS.) In
fact, as @Wessie discovered, using the result of `syspath` with `ancestry` leads
to incorrect behavior. I checked and this should not currently happen anywhere,
but these docstring changes make that requirement explicit.

beets/util/__init__.py

@@ -116,8 +116,11 @@ def normpath(path):
 def ancestry(path, pathmod=None):
     """Return a list consisting of path's parent directory, its
     grandparent, and so on. For instance:
+
        >>> ancestry('/a/b/c')
        ['/', '/a', '/a/b']
+
+    The argument should *not* be the result of a call to `syspath`.
     """
     pathmod = pathmod or os.path
     out = []
@@ -226,8 +229,11 @@ def prune_dirs(path, root=None, clutter=('.DS_Store', 'Thumbs.db')):
 
 def components(path, pathmod=None):
     """Return a list of the path components in path. For instance:
+
        >>> components('/a/b/c')
        ['a', 'b', 'c']
+
+    The argument should *not* be the result of a call to `syspath`.
     """
     pathmod = pathmod or os.path
     comps = []

docs/changelog.rst

@@ -62,6 +62,8 @@ Changelog
 * Fix for changing date fields (like "year") with the :ref:`modify-cmd`
   command.
 * Fix a crash when input is read from a pipe without a specified encoding.
+* Fix some problem with identifying files on Windows with Unicode directory
+  names in their path.
 * Add a human-readable error message when writing files' tags fails.
 * Changed plugin loading so that modules can be imported without
   unintentionally loading the plugins they contain.