path-walk API: avoid adding a root tree more than once

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.

Signed-off-by: Johannes Schindelin <[email protected]>

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