archiver/archiver: Use Index.Has() instead of Index.Lookup() in isKnownBlob

Index.Has() is a faster then Index.Lookup() for checking if a blob exists
in the index.  As the returned data is never used, this avoids a ton
of allocations.

internal/archiver/archiver.go

@@ -86,8 +86,7 @@ func (arch *Archiver) isKnownBlob(id restic.ID, t restic.BlobType) bool {
 
 	arch.knownBlobs.Insert(id)
 
-	_, found := arch.repo.Index().Lookup(id, t)
-	if found {
+	if arch.repo.Index().Has(id, t) {
 		return true
 	}