editor: prefer 'intermediate-source' extra to use for HGREVISION environment variable Revision 51930a7180bd ('graft: record intermediate grafts in extras') introduced 'intermediate-source' extra which refers to the closest graft source. As 'intermediate-source' extra provides more detailed information about the source changeset than 'source' one, it is better to prefer the first one to use as a value of HGREVISION environment variable for an editor.