objectstore: snapshot takes Object not path

Refactor the `ObjectStore.snapshot` method to take an `Object` not
a plain filesystem tree, so the latter is more encapsulated from
the ObjectStore user (e.g. the pipeline) and prepares a unified
code-path for `snapshot` and `commit` in the future.

test/test_objectstore.py

@@ -72,7 +72,7 @@ class TestObjectStore(unittest.TestCase):
         with object_store.new("b") as tree:
             p = Path(f"{tree.path}/A")
             p.touch()
-            object_store.snapshot(tree.path, "a")
+            object_store.snapshot(tree, "a")
             p = Path(f"{tree.path}/B")
             p.touch()