Use fsdecode for template substitution

The idea in this PR is to converge on Python's `fsdecode` and `fsencode`
for argument manipulation. This seems to match up with the Python
standard library's assumptions: namely, on Windows, we use `fsdecode` to
get back to Unicode strings:
54bbb5e336/Lib/subprocess.py (L561)

So let's start by dropping this utility and going straight for
`fsdecode` here to match.

beets/util/__init__.py

@@ -1092,21 +1092,3 @@ def lazy_property(func):
         return value
 
     return wrapper
-
-
-def decode_commandline_path(path):
-    """Prepare a path for substitution into commandline template.
-
-    On Python 3, we need to construct the subprocess commands to invoke as a
-    Unicode string. On Unix, this is a little unfortunate---the OS is
-    expecting bytes---so we use surrogate escaping and decode with the
-    argument encoding, which is the same encoding that will then be
-    *reversed* to recover the same bytes before invoking the OS. On
-    Windows, we want to preserve the Unicode filename "as is."
-    """
-    # On Python 3, the template is a Unicode string, which only supports
-    # substitution of Unicode variables.
-    if platform.system() == 'Windows':
-        return path.decode(_fsencoding())
-    else:
-        return path.decode(arg_encoding(), 'surrogateescape')

beetsplug/convert.py

@@ -14,7 +14,7 @@
 
 """Converts tracks or albums to external directory
 """
-from beets.util import par_map, decode_commandline_path, arg_encoding
+from beets.util import par_map, arg_encoding
 
 import os
 import threading
@@ -214,8 +214,8 @@ class ConvertPlugin(BeetsPlugin):
             self._log.info('Encoding {0}', util.displayable_path(source))
 
         command = command.decode(arg_encoding(), 'surrogateescape')
-        source = decode_commandline_path(source)
-        dest = decode_commandline_path(dest)
+        source = os.fsdecode(source)
+        dest = os.fsdecode(dest)
 
         # Substitute $source and $dest in the argument list.
         args = shlex.split(command)

beetsplug/duplicates.py

@@ -16,11 +16,12 @@
 """
 
 import shlex
+import os
 
 from beets.plugins import BeetsPlugin
 from beets.ui import decargs, print_, Subcommand, UserError
 from beets.util import command_output, displayable_path, subprocess, \
-    bytestring_path, MoveOperation, decode_commandline_path
+    bytestring_path, MoveOperation
 from beets.library import Item, Album
 
 
@@ -200,7 +201,7 @@ class DuplicatesPlugin(BeetsPlugin):
         output as flexattr on a key that is the name of the program, and
         return the key, checksum tuple.
         """
-        args = [p.format(file=decode_commandline_path(item.path))
+        args = [p.format(file=os.fsdecode(item.path))
                 for p in shlex.split(prog)]
         key = args[0]
         checksum = getattr(item, key, False)