test: correct detection of UTF8_NFD_TO_NFC for APFS

On HFS (which is the default Mac filesystem prior to High Sierra), unicode names are "decomposed" before recording. On APFS, which appears to be the new default filesystem in Mac OS High Sierra, filenames are recorded as specified by the user. APFS continues to allow the user to access it via any name that normalizes to the same thing. This difference causes t0050-filesystem.sh to fail two tests. Improve the test for a NFD/NFC in test-lib.sh: Test if the same file can be reached in pre- and decomposed unicode. Reported-By: Elijah Newren <newren@gmail.com> Signed-off-by: Torsten Bögershausen <tboegi@web.de> Tested-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>