update a listing's file don't update mother page

[jjgomera]

Hi,

if it updates a file in listing directory, nikola build only rebuilds listing outpot, the page/s where that listing is used dont update. Its necessary same dummy change in it to force a nikola build update of that "mother" page.

Saludos

[ralsina]

The listings directive doesn't add the listing source to the rest dependencies. Need to figure out how to do that.

[ralsina]

Can you check if #725 fixes it? I think it does but I did just a very light test.