t2004: standardize file naming in symlink test

Update "symlink" test to use the common file naming scheme so that its
temporary files can be cleaned up by the "rm -f path*" idiom employed by
other tests in this script.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2004-checkout-cache-temp.sh

@@ -197,13 +197,13 @@ test_expect_success 'checkout --temp within subdir' '
 
 test_expect_success 'checkout --temp symlink' '
 	rm -f path* .merge_* actual .git/index &&
-	test_ln_s_add b a &&
+	test_ln_s_add path7 path6 &&
 	git checkout-index --temp -a >actual &&
 	test_line_count = 1 actual &&
-	test $(cut "-d	" -f2 actual) = a &&
+	test $(cut "-d	" -f2 actual) = path6 &&
 	p=$(cut "-d	" -f1 actual) &&
 	test -f $p &&
-	test $(cat $p) = b
+	test $(cat $p) = path7
 '
 
 test_done