pipeline: separate object creation from access

Check for existing checkpoint in `Pipeline.build_stages` by trying to
get the object, instead of just checking for its existence. Later, if
no checkpoints were found, i.e. `tree` is `None`, create a new object.
This avoids mixing of new object creation and object access.

osbuild/pipeline.py

@@ -299,14 +299,17 @@ class Pipeline:
         # Not in the store yet, need to actually build it, but maybe
         # an intermediate checkpoint exists: Find the last stage that
         # already exists in the store and use that as the base.
-        tree = object_store.new()
+        tree = None
         todo = collections.deque()
         for stage in reversed(self.stages):
-            if object_store.contains(stage.id):
-                tree.base = stage.id
+            tree = object_store.get(stage.id)
+            if tree:
                 break
             todo.append(stage)  # append right side of the deque
 
+        if not tree:
+            tree = object_store.new()
+
         # If two run() calls race each-other, two trees will get built
         # and it is nondeterministic which of them will end up
         # referenced by the `tree_id` in the content store if they are