/usr/bin/git-setup is in git-extras 1.9.1-2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | #!/bin/sh
dir=$(test -z "$*" && echo "." || echo "$*")
mkdir -p "$dir" \
&& cd "$dir" \
&& git init \
&& git add . \
&& git commit -m 'Initial commit'
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