Adding data sources in graft not working

[leoyvens]

It's a reasonable use case for grafting to add a data source with a start block larger than the graft block. However the recent change to a private data sources table broke this, because it assumes that the manifest index of the templates is the same as in the source subgraph, and adding a data source will increase the manifest index. This can be fixed by translating the manifest indexes when grafting.