_diff_system reached complexity 42, and its hash-based rename fallback
is the part that stands alone: a platform is free to call a file
whatever it likes, so a name matching nothing is not yet a gap, and
counting one file as both missing and extra invents a discrepancy.
That step is now its own function at complexity 31, with the tests it
never had: pairing on any of the three digests, case folding, non-string
values, and the case where two files simply have no hashes and so are
not evidence of anything. diff_truth output is unchanged.
Abdessamad Derraz