index: add AddToSupersedes()
index: add AddToSupersedes()

jayme-github/restic,kurin/restic,episource/restic,ar-jan/restic,restic/restic,jayme-github/restic,mappu/restic,howeyc/restic,restic/restic,arithmetric/restic,byo/restic,fawick/restic,ckemper67/restic,ar-jan/restic,middelink/restic,stuertz/restic,kurin/restic,middelink/restic,fawick/restic,byo/restic,stuertz/restic,bchapuis/restic