Make black output more informative

[RMeli]

[...]

The feedback from linters/black is close to useless to me as it just says "2 files would be reformatted, 218 files would be left unchanged." without telling me which one. (And please don't make me read the list of 200 files that it touched to find the one that does not say "already well formatted, good job."...).

Originally posted by @orbeckst in #5423 (comment)

[orbeckst]

(I learned to search for the word "would" in the output to see the files that it "would have changed".)

[RMeli]

I do the same, but it's annoying. =)