ostree: Use G_OPTION_ARG_FILENAME where appropriate

ostree: Use G_OPTION_ARG_FILENAME where appropriate

Instead of using G_OPTION_ARG_STRING, use G_OPTION_ARG_FILENAME, which
handles filename encoding conversion differently from the locale
conversion which G_OPTION_ARG_STRING. This will fix argument handling on
systems where the filename encoding is not the same as the locale
encoding (which is fairly unlikely since most systems use UTF-8).

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #810
Approved by: cgwalters