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.

cmd/restic/integration_test.go

@@ -18,6 +18,7 @@ import (
 	"github.com/restic/restic/backend"
 	"github.com/restic/restic/debug"
 	"github.com/restic/restic/filter"
+	"github.com/restic/restic/repository"
 	. "github.com/restic/restic/test"
 )
 
@@ -683,3 +684,8 @@ func TestRebuildIndex(t *testing.T) {
 		}
 	})
 }
+
+func TestRebuildIndexAlwaysFull(t *testing.T) {
+	repository.IndexFull = func(*repository.Index) bool { return true }
+	TestRebuildIndex(t)
+}