If a .git directory is found in a web application you can download all the content using wget -r http://web.com/.git. Then, you can see the changes made by using git diff.
The tool https://github.com/cve-search/git-vuln-finder can be used to search for CVEs and security vulnerability messages inside commits messages.
The tool https://github.com/michenriksen/gitrob search for sensitive data in the repositories of an organisations and its employees.
Repo security scanner is a command line-based tool that was written with a single goal: to help you discover GitHub secrets that developers accidentally made by pushing sensitive data. And like the others, it will help you find passwords, private keys, usernames, tokens and more.
TruffleHog searches through GitHub repositories and digs through the commit history and branches, looking for accidentally committed secrets
Here you can find an study about github dorks: https://securitytrails.com/blog/github-dorks