objectstore: change Object.move to .store_tree

Now that Object manages its work directory itself, re-create the
latter when the its tree is moved, i.e. when the object is being
committed to the store. This means that after the object has been
written to the store it is in the same state is if it was new and
can be used in the very same way.
If the move itself fails (the rename(2) fails), the tree and its
contents is cleaned up with the reset of the work directory.
Rename the `move` method to `store_tree` to better reflect how the
method should be used, i.e. to store the tree corresponding to the
Object instance.

osbuild/objectstore.py

@@ -62,15 +62,16 @@ class Object:
         finally:
             os.close(fd)
 
-    def move(self, destination: str):
-        """Move the object to destination
+    def store_tree(self, destination: str):
+        """Store the tree at destination and reset itself
 
-        Does so atomically by using rename(2). If the
-        target already exist, use that instead
+        Moves the tree atomically by using rename(2). If the
+        target already exist, does nothing. Afterwards it
+        resets itself and can be used as if it was new.
         """
         with suppress_oserror(errno.ENOTEMPTY, errno.EEXIST):
             os.rename(self.path, destination)
-        self._tree = destination
+        self.reset()
 
     def reset(self):
         self.cleanup()
@@ -194,7 +195,7 @@ class ObjectStore:
         # will always produce the same content hash, but that is not
         # guaranteed. If an object with the same treesum already exist, us
         # the existing one instead
-        obj.move(f"{self.objects}/{treesum_hash}")
+        obj.store_tree(f"{self.objects}/{treesum_hash}")
 
         # symlink the object_id (config hash) in the refs directory to the
         # treesum (content hash) in the objects directory. If a symlink by