t3910: use the UTF8_NFD_TO_NFC test prereq

Besides reusing the new test prerequisite, this fixes also the issue
that the current output is not TAP compliant and produces the output "no
reason given" [for skipping].

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3910-mac-os-precompose.sh

@@ -7,24 +7,16 @@ test_description='utf-8 decomposed (nfd) converted to precomposed (nfc)'
 
 . ./test-lib.sh
 
+if ! test_have_prereq UTF8_NFD_TO_NFC
+then
+	skip_all="filesystem does not corrupt utf-8"
+	test_done
+fi
+
+# create utf-8 variables
 Adiarnfc=`printf '\303\204'`
 Adiarnfd=`printf 'A\314\210'`
 
-# check if the feature is compiled in
-mkdir junk &&
->junk/"$Adiarnfc" &&
-case "$(cd junk && echo *)" in
-	"$Adiarnfd")
-	test_nfd=1
-	;;
-	*)	;;
-esac
-rm -rf junk
-
-
-if test "$test_nfd"
-then
-	# create more utf-8 variables
 Odiarnfc=`printf '\303\226'`
 Odiarnfd=`printf 'O\314\210'`
 AEligatu=`printf '\303\206'`
@@ -157,8 +149,5 @@ then
 	precomposeunicode=`git config core.precomposeunicode` &&
 	test "$precomposeunicode" = "true"
 '
-else
-	 say "Skipping nfc/nfd tests"
-fi
 
 test_done