my local experiments showed any tricky scheme I could come up beaten by plain large window zstd. both on speed and resulting size (memory needed for the window is another matter completely). zstd also was beating the git xdelta+deflate.

The most "free lunch" actually is compressing fast export (or similar format) — if the goal is packing repo as small as possible, then all the SHA in commits and tree objects are derivative incompressible data, and plain references like in fast export can even halve archive size