merge: use original file extension for temporary files
Some merge tools (like Araxis?) can pick merge mode based on the file
extension. That didn't work well when temporary files were given random
suffixes. It seems to work better when the random part is before the extension.
As usual, when using $output, $local will have the .orig extension. That could
perhaps be the subject of another change another day.
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors