Compression and dictionaries

dictionaries as a way of achieving significant compression. If so, since we zlib compress each entry in a pack individually, are there many copies of very similar dictionaries in the pack?

Some compression schemes support being initialized with a fixed dictionary and sharing it over all entries. A fixed dictionary could be built by analysing a large pack file. Sharing a compression dictionary would probably be a win for me since I have 1M+ entries in a pack.

Poking around in the zlib it appears that zlib supports precomputed dictionaries. http://www.zlib.net/manual.html

  int deflateSetDictionary (z_streamp strm, const Bytef *dictionary,
uInt dictLength);