b2: Repeat deleting until all file versions are removed

When hard deleting the latest file version on B2, this uncovers earlier
versions. If an upload required retries, multiple version might exist
for a file. Thus to reliably delete a file, we have to remove all
versions of it.

internal/backend/b2/b2.go

@@ -274,16 +274,24 @@ func (be *b2Backend) Remove(ctx context.Context, h restic.Handle) error {
 	be.sem.GetToken()
 	defer be.sem.ReleaseToken()
 
+	// the retry backend will also repeat the remove method up to 10 times
+	for i := 0; i < 3; i++ {
 		obj := be.bucket.Object(be.Filename(h))
 		err := obj.Delete(ctx)
+		if err == nil {
+			// keep deleting until we are sure that no leftover file versions exist
+			continue
+		}
 		// consider a file as removed if b2 informs us that it does not exist
 		if b2.IsNotExist(err) {
 			return nil
 		}
-
 		return errors.Wrap(err, "Delete")
 	}
 
+	return errors.New("failed to delete all file versions")
+}
+
 type semLocker struct {
 	sema.Semaphore
 }