py3: add os.fsdecode() as pycompat.fsdecode()

We need to use os.fsdecode() but this was not present in Python 2. So added
the function in pycompat.py

mercurial/pycompat.py

@@ -36,6 +36,7 b' if ispy3:'
     import functools
     import os
     fsencode = os.fsencode
+    fsdecode = os.fsdecode
 
     def sysstr(s):
         """Return a keyword str to be passed to Python functions such as
@@ -76,6 +77,11 b' else:'
             raise TypeError(
                 "expect str, not %s" % type(filename).__name__)
 
+    # In Python 2, fsdecode() has a very chance to receive bytes. So it's
+    # better not to touch Python 2 part as it's already working fine.
+    def fsdecode(filename):
+        return filename
+
 stringio = io.StringIO
 empty = _queue.Empty
 queue = _queue.Queue