Refactor getOverlayDatabaseMode and add new disablement reason

Author: henrymercer

lib/init-action.js

@@ -1165,6 +1165,7 @@ test(
   {
     overlayDatabaseMode: OverlayDatabaseMode.None,
     useOverlayDatabaseCaching: false,
+    disabledReason: OverlayDisabledReason.NotPullRequestOrDefaultBranch,
   },
 );