Feature summary
for example, i just uploaded https://live.staticflickr.com/7016/6741180925_696ba73388_o_d.jpg to a commons file.
https://commons.wikimedia.org/w/index.php?title=File%3AEmission_of_Greenhouse_Gases.jpg&diff=prev&oldid=742356206&diffmode=source

i didnt write anything in "File changes:" in the upload form, so the "Comment" appears blank.

it would be useful if the comment will include the source url for this particular kind of overwrite-upload when it is otherwise empty or only whitespace.

Benefits (why should this be implemented?):

1. provides basic info when the comment is otherwise blank.
2. saves uploader time from writing a comment if the upload is self explanatory (like the example is the flickr original).

PS:

1. this is not commons-only, but i cant find a tag for the "upload form".
2. for uploads from local (not from source url), some kind of automatic comment that gives more info will also be good.