Starting from a single file, A, if you create B as a copy of A (and
possibly make some edit) and then make extensive change to A, you
will see:
$ git diff -C --name-status
C89 A B
M A
which is expected. However, if you ask the same question in a
different way, you see this:
$ git diff -B -M --name-status
R89 A B
M100 A
telling us that A was rename-edited into B (as if "A will no longer
exist as the result") and at the same time A itself was extensively
edited.
In this case, because the resulting tree still does have file A
(even if it has contents vastly different from the original), we
should use "C"opy, not "R"ename, to avoid hinting that A somehow
goes away.
Two existing tests were depending on the wrong behaviour, and fixed.