Fix: whole knowledge graph lost after removing any document in the knowledge base (#7151)

### What problem does this PR solve?

When you removed any document in a knowledge base using knowledge graph,
the graph's `removed_kwd` is set to "Y".
However, in the function `graphrag.utils.get_gaph`, `rebuild_graph`
method is passed and directly return `None` while `removed_kwd=Y`,
making residual part of the graph abandoned (but old entity data still
exist in db).

Besides, infinity instance actually pass deleting graph components'
`source_id` when removing document. It may cause wrong graph after
rebuild.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

graphrag/general/index.py

@@ -204,7 +204,7 @@ async def merge_subgraph(
 ):
     start = trio.current_time()
     change = GraphChange()
-    old_graph = await get_graph(tenant_id, kb_id)
+    old_graph = await get_graph(tenant_id, kb_id, subgraph.graph["source_id"])
     if old_graph is not None:
         logging.info("Merge with an exiting graph...................")
         tidy_graph(old_graph, callback)