t3600: use test_line_count() where possible

Since we have a helper function that can test the number of lines in a
file that gives better debugging information on failure, use
test_line_count() to test the number of lines.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3600-rm.sh

@@ -113,9 +113,10 @@ test_expect_success '"rm" command printed' '
 	echo frotz >test-file &&
 	git add test-file &&
 	git commit -m "add file for rm test" &&
-	git rm test-file >rm-output &&
-	test $(grep "^rm " rm-output | wc -l) = 1 &&
-	rm -f test-file rm-output &&
+	git rm test-file >rm-output.raw &&
+	grep "^rm " rm-output.raw >rm-output &&
+	test_line_count = 1 rm-output &&
+	rm -f test-file rm-output.raw rm-output &&
 	git commit -m "remove file from rm test"
 '