Tested with QEMU at db596ae19040574e41d086e78469014191d7d7fc.
$ git clone --mirror https://gitlab.com/qemu-project/qemu.git
$ ditto qemu.git qemu-nocomp.git
$ gtar -c qemu.git > qemu.git.tar
$ pushd qemu-nocomp.git
$ # We're going to repack the git repo into a single pack file with no zlib compression
$ # so later compression can compress the uncompressed pack file better.
$ git config --add --int pack.compression 0
$ git config --add --int core.compression 0
$ # Does this result in 100% perfect mirror? Not entirely sure...
$ git repack -k -F -f -a -d --window-memory=0 -d -b --pack-kept-objects
$ popd
$ gtar -c qemu-nocomp.git > qemu-nocomp.git.tar
$ # 7zz -version
$ # 7-Zip (z) 23.01 (arm64) : Copyright (c) 1999-2023 Igor Pavlov : 2023-06-20
$ # 64-bit arm_v:8 locale=en_US.UTF-8 Threads:10 OPEN_MAX:256, ASM
$ 7zz a -bt -slp -ssw -bb3 -mmt1 -mx9 -ms=on -md=1024m -m0=lzma2 qemu-nocomp.git.tar.7znomt qemu-nocomp.git.tar
$ 7zz a -bt -slp -ssw -bb3 -mmt10 -mx9 -ms=on -md=1024m -m0=lzma2 qemu-nocomp.git.tar.7z qemu-nocomp.git.tar
$ 7zz a -bt -slp -ssw -bb3 -mmt1 -mx9 -ms=on -md=1024m -m0=lzma2 qemu.git.tar.7znomt qemu.git.tar
$ 7zz a -bt -slp -ssw -bb3 -mmt10 -mx9 -ms=on -md=1024m -m0=lzma2 qemu.git.tar.7z qemu.git.tar
$ # p7zip -version
$ # 7-Zip [64] 17.04 : Copyright (c) 1999-2021 Igor Pavlov : 2017-08-28
$ # p7zip Version 17.04 (locale=utf8,Utf16=on,HugeFiles=on,64 bits,10 CPUs LE)
$ 7z a -bt -slp -ssw -bb3 -mmt1 -mx9 -ms=on -md=1024m -m0=lzma2 qemu-nocomp.git.tar.7znomt qemu-nocomp.git.tar
$ 7z a -bt -slp -ssw -bb3 -mmt10 -mx9 -ms=on -md=1024m -m0=lzma2 qemu-nocomp.git.tar.7z qemu-nocomp.git.tar
$ 7z a -bt -slp -ssw -bb3 -mmt1 -mx9 -ms=on -md=1024m -m0=lzma2 qemu.git.tar.7znomt qemu.git.tar
$ 7z a -bt -slp -ssw -bb3 -mmt10 -mx9 -ms=on -md=1024m -m0=lzma2 qemu.git.tar.7z qemu.git.tar
$ # xz --version
$ # xz (XZ Utils) 5.6.0
$ # liblzma 5.6.0
$ xz -z -k -9 -e -vv qemu-nocomp.git.tar
$ xz -z -k -9 -e -vv qemu.git.tar
$ # zstd --version
$ # *** Zstandard CLI (64-bit) v1.5.5, by Yann Collet ***
$ zstd -vvv --progress --no-row-match-finder -M$((1024*32)) -T0 qemu-nocomp.git.tar -o qemu-nocomp.git.tar.zstdstd
$ zstd -vvv --progress --ultra -22 --no-row-match-finder -M$((1024*32)) -T0 qemu-nocomp.git.tar -o qemu-nocomp.git.tar.zstdmt
$ zstd -vvv --progress --ultra -22 --no-row-match-finder -M$((1024*32)) qemu-nocomp.git.tar -o qemu-nocomp.git.tar.zstd
$ zstd -vvv --progress --no-row-match-finder -M$((1024*32)) -T0 qemu-nocomp.git.tar -o qemu-nocomp.git.tar.zstdstd
$ zstd -vvv --progress --ultra -22 --no-row-match-finder -M$((1024*32)) -T0 qemu.git.tar -o qemu.git.tar.zstdmt
$ zstd -vvv --progress --ultra -22 --no-row-match-finder -M$((1024*32)) qemu.git.tar -o qemu.git.tar.zstd
$ ls -lh *.tar*
663M qemu-nocomp.git.tar
350M qemu.git.tar
340M qemu.git.tar.zstdstd
332M qemu.git.tar.xz
330M qemu.git.tar.zstd
330M qemu.git.tar.zstdmt
329M qemu.git.tar.7z
329M qemu.git.tar.7znomt
329M qemu.git.tar.p7z
329M qemu.git.tar.p7znomt
268M qemu-nocomp.git.tar.zstdstd
224M qemu-nocomp.git.tar.xz
223M qemu-nocomp.git.tar.zstd
223M qemu-nocomp.git.tar.zstdmt
209M qemu-nocomp.git.tar.7z
209M qemu-nocomp.git.tar.7znomt
$ ls -lS *.tar*
695367680 qemu-nocomp.git.tar
367001600 qemu.git.tar
356934615 qemu.git.tar.zstdstd
348128456 qemu.git.tar.xz
346519601 qemu.git.tar.zstd
346519601 qemu.git.tar.zstdmt
344577602 qemu.git.tar.7z
344577602 qemu.git.tar.7znomt
344575339 qemu.git.tar.p7z
344575339 qemu.git.tar.p7znomt
281105905 qemu-nocomp.git.tar.zstdstd
234952688 qemu-nocomp.git.tar.xz
233654293 qemu-nocomp.git.tar.zstd
233654293 qemu-nocomp.git.tar.zstdmt
218819332 qemu-nocomp.git.tar.7z
218814936 qemu-nocomp.git.tar.7znomt
Write stript to handle submodules, automatic pack decompression, and pack recompression after uncompression.