Avoid dropping is_complete column for backward compatibility

Co-authored-by: Antonio Scandurra <antonio@zed.dev>

crates/collab/migrations.sqlite/20221109000000_test_schema.sql

@@ -57,6 +57,7 @@ CREATE TABLE "worktrees" (
     "abs_path" VARCHAR NOT NULL,
     "visible" BOOL NOT NULL,
     "scan_id" INTEGER NOT NULL,
+    "is_complete" BOOL NOT NULL DEFAULT FALSE,
     "completed_scan_id" INTEGER NOT NULL,
     PRIMARY KEY(project_id, id)
 );

crates/collab/migrations/20230103200902_replace_is_completed_with_completed_scan_id.sql

@@ -1,3 +1,3 @@
 ALTER TABLE worktrees
-    DROP COLUMN is_complete,
+    ALTER COLUMN is_complete SET DEFAULT FALSE,
     ADD COLUMN completed_scan_id INT8;