filter-repo (python3): add a decode() function

We need a function to transform byte strings into unicode strings for
printing error messages and occasional other uses.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -115,6 +115,10 @@ def date_to_string(dateobj):
   return('{} {}'.format(int(_timedelta_to_seconds(dateobj - epoch)),
                         dateobj.tzinfo.tzname(0)))
 
+def decode(bytestr):
+  'Try to convert bytestr to utf-8 for outputting as an error message.'
+  return bytestr.decode('utf-8', 'backslashreplace')
+
 class PathQuoting:
   _unescape = {'a': '\a',
                'b': '\b',