When the user provides a filename without an extension in the save
dialog, we add one for them, update the filename in the dialog, and
retry.  However, the updated filename is made up of only the
basename, leaving out the dirname part, if specified.  This means
that if the user enters "/somedir/somefile", the new filename
becomes "somefile.xcf", which refers to the current directory,
instead of "somedir".

Fix this by maintaining the dirname when adding a file extension.