In some cases, updating the artifact link field could move an autocomputed field from autocomputed value to manual value (for instance with remaining_effort).

To reproduce the initial issue:

* Need the following tracker hierarchy: Story parent of Task
* Create a User Story and submit
* Edit the the Linked Artifact field of the story in step 1
* Select "Child" as the Link Type and create a task
* Enter a value in the "Remaing Effort" field of the task and save changes
* Create another task repeating steps 3 & 4. Submit the story to save the changes
* Verify that the "Remaining Effort" in the story displays the "Auto Compute" value
* Now edit the Linked Artifact field of the same story used in steps above
* Create a new task select "Child" as the Link Type
* Enter a value in the "Remaining Effort" field of the task submit the task and the story.
* Check the "Remaining Effort" field in the story (it no longer states auto compute) and check the history which now says it was changed from auto comput to . . .