t7400: barf if git-submodule removes or replaces a file

The test for an unmolested file wouldn't fail properly if the file had been
removed or replaced by something other than a regular file. This fixes it.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -72,7 +72,7 @@ test_expect_success 'update should fail when path is used by a file' '
 	then
 		echo "[OOPS] update should have failed"
 		false
-	elif test -f lib && test "$(cat lib)" != "hello"
+	elif test "$(cat lib)" != "hello"
 	then
 		echo "[OOPS] update failed but lib file was molested"
 		false