t4124 (apply --whitespace): use test_might_fail

Use test_might_fail instead of ignoring the exit status from git
config --unset, and let the exit status propagate past rm -f (which
does not fail on ENOENT).  Otherwise bugs that lead git config to
crash would not be detected when this test runs.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4124-apply-ws-rule.sh

@@ -176,9 +176,8 @@ test_expect_success 'trailing whitespace & no newline at the end of file' '
 '
 
 test_expect_success 'blank at EOF with --whitespace=fix (1)' '
-	: these can fail depending on what we did before
-	git config --unset core.whitespace
-	rm -f .gitattributes
+	test_might_fail git config --unset core.whitespace &&
+	rm -f .gitattributes &&
 
 	{ echo a; echo b; echo c; } >one &&
 	git add one &&