git-diff-files: add page

[adipopbv]

• The page (if new), does not already exist in the repository.
• The page is in the correct platform directory (common/, linux/, etc.)
• The page has 8 or fewer examples.
• The PR title conforms to the recommended templates.
• The page follows the content guidelines.
• The page description includes a link to documentation or a homepage (if applicable).

Version of the command being documented (if known):

Tested with git 2.33.0

[marchersimon]

Even though you used the description from the docs, I don't think it's quite correct. It doesn't compare files like with git diff, but only shows their sha1 hashes and modes.

[marchersimon]

Thanks @adipopbv