save: make the py3 save commit message args match py2

We shouldn't use sys.argv, it's missing the actual arguments
now. Also, encode it but drop the "b" on the bytes (if present,
i.e. python3) to make this look the same as on python2.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
[rlb@defaultvalue.org: make formatting decision via py_maj; adjust
 commit message]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>