Add test for corner case
Add test for corner case

It was observed that a restic repository still contained overlapping
indexes after `rebuild-index` has been called. This is caused by
instantly forgetting that blobs have already been saved once a full
index has been written during index rebuilding.

This commit adds a (failing) test that shows the behaviour.

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