postinst is in git-daemon-sysvinit 1:1.9.1-1.
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 6 7 8 9 10 11 12 13 14 15 16 17 18 19 | #!/bin/sh
set -e
test "$1" = 'configure' || exit 0
getent passwd gitdaemon >/dev/null || \
  adduser --system --home /nonexistent --no-create-home gitdaemon
if [ -x "/etc/init.d/git-daemon" ]; then
	# enable git-daemon service
	update-rc.d git-daemon defaults >/dev/null
	# restart git-daemon service if it was running
	if [ -x "`which invoke-rc.d 2>/dev/null`" ]; then
		invoke-rc.d git-daemon start || exit $?
	else
		/etc/init.d/git-daemon start || exit $?
	fi
fi
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