There is an example of how to use mosquitto_sub on that page, but I find the -v option then requires using only the second column in the log, so I like to take it out. Here is what I would use:
mosquitto_sub --cafile ~/.centos-server-ca.cert --cert ~/.centos.cert --key ~/.centos.cert -h mqtt.git.centos.org -p 8883 --id <uniq-id>
--disable-clean-session --keepalive -t git.centos.org/# >><filepath>
Once you have the default log going, you can parse the log as json .. I like to use jq. Here is an example jq query that will create an output that looks like this for branches that begin with c7 in the log.
cat mosquitto_sub.log.txt | jq -j ' . | select(.branch | startswith("c7")) | .repo.name," ",.repo.date_modified," ",.end_commit," ",.branch,"\n"' 2>/dev/null| sed -e 's,",,g'
The output will be similar to this if an rpm update is found:
mod_auth_mellon 1553623560 8b0a0c922f1f1b030200998f3bd069f771b4b4a5 c7
The information is the RPM Name, Date Modfied, Git Commit ID, and Branch for the commit.