chmod: Fix chmod -c --reference reporting when no change is made

[mtimaN]

Fix for issue #6009. I changed the code so that the old permission octal is retrieved in "full form" (6 octal digits). It is afterwards set to 4 digits when displayed.

Beforehand, 2 identical permissions were marked as different because of the different formats, resulting in incorrect displays for -c.

[BenWiederhake]

Is it possible to add a test for this? :)

[mtimaN]

@BenWiederhake yep! It also failed a test beforehand so I kind of flipped the logic: made the reference permission 4 octals long, just as the others.

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[cakebaker]

Thanks for your PR!