You don't even need to write your key/value in a file. Git accept committing empty commit. and allow any message to be committed with it so content of your database can be the git commit message.
I wrote about that concept before and tried to implement it but never went to the full potential (too many side projects)
https://www.emadelsaid.com/git-as-messages-queue/https://github.com/emad-elsaid/gitmq