t6300: don't run cat-file on non-existent object

In t6300, some tests are guarded behind some prerequisites.
Thus, objects created by those tests ain't available if those
prerequisites are unsatistified.  Attempting to run "cat-file"
on those objects will run into failure.

In fact, running t6300 in an environment without gpg(1),
we'll see those warnings:

	fatal: Not a valid object name refs/tags/signed-empty
	fatal: Not a valid object name refs/tags/signed-short
	fatal: Not a valid object name refs/tags/signed-long

Let's put those commands into the real tests, in order to:

* skip their execution if prerequisites aren't satistified.
* check their exit status code

The expected value for objects with type: commit needs to be
computed outside the test because we can't rely on "$3" there.
Furthermore, to prevent the accidental usage of that computed
expected value, BUG out on unknown object's type.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6300-for-each-ref.sh

@@ -59,18 +59,23 @@ test_atom() {
 	# Automatically test "contents:size" atom after testing "contents"
 	if test "$2" = "contents"
 	then
-		case $(git cat-file -t "$ref") in
+		# for commit leg, $3 is changed there
+		expect=$(printf '%s' "$3" | wc -c)
+		test_expect_${4:-success} $PREREQ "basic atom: $1 contents:size" '
+			type=$(git cat-file -t "$ref") &&
+			case $type in
 			tag)
 				# We cannot use $3 as it expects sanitize_pgp to run
 				expect=$(git cat-file tag $ref | tail -n +6 | wc -c) ;;
 			tree | blob)
-			expect='' ;;
+				expect="" ;;
 			commit)
-			expect=$(printf '%s' "$3" | wc -c) ;;
-		esac
+				: "use the calculated expect" ;;
+			*)
+				BUG "unknown object type" ;;
+			esac &&
 			# Leave $expect unquoted to lose possible leading whitespaces
-		echo $expect >expected
-		test_expect_${4:-success} $PREREQ "basic atom: $1 contents:size" '
+			echo $expect >expected &&
 			git for-each-ref --format="%(contents:size)" "$ref" >actual &&
 			test_cmp expected actual
 		'