py3: make sure decode() first argument is str

Uses pycompat.sysstr() to make sure we uses bytes on Python 2 and unicodes on
Python 3.

Differential Revision: https://phab.mercurial-scm.org/D3279

hgext/convert/common.py

@@ -217,12 +217,13 b' class converter_source(object):'
         if isinstance(s, unicode):
             return s.encode("utf-8")
         try:
-            return s.decode(encoding).encode("utf-8")
+            return s.decode(pycompat.sysstr(encoding)).encode("utf-8")
         except UnicodeError:
             try:
                 return s.decode("latin-1").encode("utf-8")
             except UnicodeError:
-                return s.decode(encoding, "replace").encode("utf-8")
+                return s.decode(pycompat.sysstr(encoding),
+                                "replace").encode("utf-8")
 
     def getchangedfiles(self, rev, i):
         """Return the files changed by rev compared to parent[i].