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.