logging causes exceptions for iso-8859-1 filenames

[az]

this is a forward of debian bug #682837, which lives over there: http://bugs.debian.org/682837

the original submitter gets lots of exceptions because the log module doesn't deal properly with
the filenames it's supposed to log: the filename in question includes characters from
iso-8859-1 aka latin1, namely an a-circumflex, while duplicity's log module insists on plain ascii or utf-8.

here is an excerpt of the error message:
  File "/usr/lib/python2.7/dist-packages/duplicity/collections.py", line 805, in add_to_sets
    log.Debug(_("File %s is part of known set") % (filename,))
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 16: ordinal not in range(128)

[Michael Terry (mterry)]

This looks a lot like bug 989496. I'm not marking as a duplicate yet, because there may be two related faults with unicode, and this may be a good bug for the non-utf8-filenames side of things (rather than the non-utf8-translations side).