preinst is in git-core 1:2.7.4-0ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 | #!/bin/sh
set -e
test -z "$2" || dpkg --compare-versions "$2" gt '1:1.5.2~rc3-2' ||
exec update-alternatives --remove git /usr/bin/git-scm
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