'Create a new MemoryPackIndex.
:param entries: Sequence of name, idx, crc32 (sorted)
:param pack_checksum: Optional pack checksum'

def __init__(self, entries, pack_checksum=None):

    self._by_sha = {}
    for (name, idx, crc32) in entries:
        self._by_sha[name] = idx
    self._entries = entries
    self._pack_checksum = pack_checksum
