Add "do nothing" path_msg to centralize path conversions

Add "do nothing" path_msg to centralize path conversions

Add path_msg(p) and use it in cmd/bup (as a start).  The intent is to
centralize the encoding of all path values that are to be included in
strings intended for "display" (e.g. stderr, which appears to have to
be a text stream in python 3).

For now, the function will do nothing -- i.e. given the currently
enforced iso-8859-1 encoding we'll just continue to produce the
original path bytes on stderr, but we may well want to make this
configurable at some point (perhaps git's quotePath algorithm might
provide a likely option), and if nothing else, using path_msg()
everywhere makes it much easier to identify and adapt the relevant
code, whatever we decide.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>