in '''`.git/hooks/post-commit`''' 

{{{#!hook

#!/bin/sh
commit=`git log -1 --format=format:%s`
user="tomk32"
mosquitto_pub -d  -h mqtt.devlol.org --insecure -t "$user/commit" -m "$commit"
}}}

Don't forget to run '''`chmod a+x .git/hooks/post-commit`'''