path-walk API: avoid adding a root tree more than once (#5195)

When adding tree objects, we are very careful to avoid adding the same
tree object more than once. There was one small gap in that logic,
though: when adding a root tree object. Two refs can easily share the
same root tree object, and we should still not add it more than once.

Author: dscho

t/t6601-path-walk.sh

@@ -397,4 +397,26 @@ test_expect_success 'trees are reported exactly once' '
 	test_line_count = 1 out-filtered
 '
 
+test_expect_success 'trees are reported exactly once' '
+	test_when_finished "rm -rf unique-trees" &&
+	test_create_repo unique-trees &&
+	(
+		cd unique-trees &&
+		mkdir initial &&
+		test_commit initial/file &&
+
+		git switch -c move-to-top &&
+		git mv initial/file.t ./ &&
+		test_tick &&
+		git commit -m moved &&
+
+		git update-ref refs/heads/other HEAD
+	) &&
+
+	test-tool -C unique-trees path-walk -- --all >out &&
+	tree=$(git -C unique-trees rev-parse HEAD:) &&
+	grep "$tree" out >out-filtered &&
+	test_line_count = 1 out-filtered
+'
+
 test_done