tests: stop using top-level "README" and "COPYING" files

tests: stop using top-level "README" and "COPYING" files

In 459b8d22e54 (tests: do not borrow from COPYING and README from the
real source, 2015-02-15) tests that used "lib-diff.sh" (called
"diff-lib.sh" then) were made to stop relying on the top-level COPYING
file, but we still had other tests that referenced it.

Let's move them over to use the "COPYING_test_data" utility function
introduced in the preceding commit, and in the case of the one test
that needed the "README" file use a ROT 13 version of that "COPYING"
test data. That test added in afd222967c6 (Extend testing git-mv for
renaming of subdirectories, 2006-07-26) just needs more test data that's not the same as the "COPYING" test data, so a ROT 13 version will do.

This change removes the last references to ../{README,COPYING} in the
test suite.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>