gio-tool-save: Use g_output_stream_write_all instead of while
gio-tool-save: Use g_output_stream_write_all instead of while

Simplify the read-write copy code and use g_output_stream_write_all
instead of while and g_output_stream_write.

https://bugzilla.gnome.org/show_bug.cgi?id=786462

endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib