Skip small files and files without hash in index rebuild
Skip small files and files without hash in index rebuild

(cherry picked from commit 93811b36b46369631f7e32f8fd48a12176ab5717)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend