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

["Đoàn Trần Công Danh" <congdanhqx@gmail.com>]

In t6300, some tests are guarded behind some prerequisites.
Thus, objects created by those tests ain't available if those
prerequisites is 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

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
---
 t/t6300-for-each-ref.sh | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/t/t6300-for-each-ref.sh b/t/t6300-for-each-ref.sh
index 9e0214076b..65fbed2bef 100755
--- a/t/t6300-for-each-ref.sh
+++ b/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
-		tag)
-			# We cannot use $3 as it expects sanitize_pgp to run
-			expect=$(git cat-file tag $ref | tail -n +6 | wc -c) ;;
-		tree | blob)
-			expect='' ;;
-		commit)
-			expect=$(printf '%s' "$3" | wc -c) ;;
-		esac
-		# Leave $expect unquoted to lose possible leading whitespaces
-		echo $expect >expected
+		expect=$(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
+				git cat-file tag $ref >out &&
+				expect=$(<out tail -n +6 | wc -c) ;;
+			tree | blob)
+				expect="" ;;
+			commit)
+				: "use the calculated expect" ;;
+			*)
+				BUG "unknown object type" ;;
+			esac &&
+			# Leave $expect unquoted to lose possible leading whitespaces
+			echo $expect >expected &&
 			git for-each-ref --format="%(contents:size)" "$ref" >actual &&
 			test_cmp expected actual
 		'
-- 
2.33.0