log: properly handle decorations with chained tags

git log did not correctly handle decorations when a tag object referenced
another tag object that was no longer a ref, such as when the second tag was
deleted.  The commit would not be decorated correctly because parse_object had
not been called on the second tag and therefore its tagged field had not been
filled in, resulting in none of the tags being associated with the relevant
commit.

Call parse_object to fill in this field if it is absent so that the chain of
tags can be dereferenced and the commit can be properly decorated.  Include
tests as well to prevent future regressions.
Signed-off-by: Nbrian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

log-tree.c

@@ -134,6 +134,8 @@ static int add_ref_decoration(const char *refname, const unsigned char *sha1, in
 		obj = ((struct tag *)obj)->tagged;
 		if (!obj)
 			break;
+		if (!obj->parsed)
+			parse_object(obj->sha1);
 		add_name_decoration(DECORATION_REF_TAG, refname, obj);
 	}
 	return 0;

t/t4205-log-pretty-formats.sh

@@ -310,4 +310,19 @@ EOF
 	test_cmp expected actual
 '
 
+test_expect_success 'log decoration properly follows tag chain' '
+	git tag -a tag1 -m tag1 &&
+	git tag -a tag2 -m tag2 tag1 &&
+	git tag -d tag1 &&
+	git commit --amend -m shorter &&
+	git log --no-walk --tags --pretty="%H %d" --decorate=full >actual &&
+	cat <<EOF >expected &&
+6a908c10688b2503073c39c9ba26322c73902bb5  (tag: refs/tags/tag2)
+9f716384d92283fb915a4eee5073f030638e05f9  (tag: refs/tags/message-one)
+b87e4cccdb77336ea79d89224737be7ea8e95367  (tag: refs/tags/message-two)
+EOF
+	sort actual >actual1 &&
+	test_cmp expected actual1
+'
+
 test_done