tag: do not show non-tag contents with "-n"

"git tag -n" did not check the type of the object it is reading the top n
lines from. At least, avoid showing the beginning of trees and blobs when
dealing with lightweight tags that point at them.

As the payload of a tag and a commit look similar in that they both start
with a header block, which is skipped for the purpose of "-n" output,
followed by human readable text, allow the message of commit objects to be
shown just like the contents of tag objects. This avoids regression for
people who have been using "tag -n" to show the log messages of commits
that are pointed at by lightweight tags.

Test script is from Jeff King.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -95,19 +95,20 @@ static void show_tag_lines(const unsigned char *sha1, int lines)
 	buf = read_sha1_file(sha1, &type, &size);
 	if (!buf)
 		die_errno("unable to read object %s", sha1_to_hex(sha1));
-	if (!size) {
-		free(buf);
-		return;
-	}
+	if (type != OBJ_COMMIT && type != OBJ_TAG)
+		goto free_return;
+	if (!size)
+		die("an empty %s object %s?",
+		    typename(type), sha1_to_hex(sha1));
 
 	/* skip header */
 	sp = strstr(buf, "\n\n");
-	if (!sp) {
-		free(buf);
-		return;
-	}
-	/* only take up to "lines" lines, and strip the signature */
-	size = parse_signature(buf, size);
+	if (!sp)
+		goto free_return;
+
+	/* only take up to "lines" lines, and strip the signature from a tag */
+	if (type == OBJ_TAG)
+		size = parse_signature(buf, size);
 	for (i = 0, sp += 2; i < lines && sp < buf + size; i++) {
 		if (i)
 			printf("\n    ");
@@ -118,6 +119,7 @@ static void show_tag_lines(const unsigned char *sha1, int lines)
 			break;
 		sp = eol + 1;
 	}
+free_return:
 	free(buf);
 }
 

t/t7004-tag.sh

@@ -585,6 +585,19 @@ test_expect_success \
 	test_cmp expect actual
 '
 
+test_expect_success 'annotations for blobs are empty' '
+	blob=$(git hash-object -w --stdin <<-\EOF
+	Blob paragraph 1.
+
+	Blob paragraph 2.
+	EOF
+	) &&
+	git tag tag-blob $blob &&
+	echo "tag-blob        " >expect &&
+	git tag -n1 -l tag-blob >actual &&
+	test_cmp expect actual
+'
+
 # subsequent tests require gpg; check if it is available
 gpg --version >/dev/null 2>/dev/null
 if [ $? -eq 127 ]; then