fix: dead loop when importing updates

[zxch3n]

Fix the dead loop issue here: https://gist.github.com/sunflowerdeath/c0e2b46b6f5d2e32d368f8e04f730237

Reason for the Dead Loop in find_common_ancestor

Original Assumptions

1. Dependency Assumption

   If a DagNode is depended upon by other nodes, the direction of this dependency will only point to before the end of the node. This assumption is used in system design.

2. Node Overlap Assumption

   DagNodes do not overlap with each other. When retrieving DagNodes from two different positions:

   • They are either completely identical
   • Or they do not overlap at all
   • If there is overlap, it means they are the same node

Current Issues

1. Issues Caused by Lazy Loading

   Due to the use of lazy loading, new situations have arisen. Initially:

   • A certain DagNode is not depended upon by other DagNodes
   • Therefore, the initially retrieved form is complete

2. Impact of Subsequent Loading

   After loading additional DagNodes:

   • The newly loaded DagNode may depend on the internal position of the previously complete DagNode
   • This causes the originally complete DagNode to be split into multiple smaller DagNodes

3. Violation of Original Assumptions

   In this case, when retrieving the content of the original DagNode again:

   • Other DagNode dependencies may appear in the middle
   • There may be overlaps on the DagNode
   • This violates the original design assumptions

Fix Approach

Remove the assumption in the original implementation that "overlapping DagNodes will not occur." When overlapping DagNodes are found, retain the shorter one and remove the longer one.