feat(black): display diff of reformatted files

When black decides to reformat one or many files, fixing those would
require the user to locally install black, with the exact same version
that was used in the pipeline, and run black locally.

This can be tedious when the diff is very minor and just need a simple
local edit to fix.

Instruct black to display the diff so that it is visible in the job's
build log. The user will then be able to decide whether they can just
manually tweak their source code for small diffs, or if they need to
go all the way to install and run black locally for bigger diffs.

Signed-off-by: Yann E. MORIN <yann.morin@orange.com>