"Garbage collect" old commits in git repository to free disk space

Hello, is there safe way to garbage collect old commits from git repository? Lets say that i want to always keep only last 100 commits and throw everything older away. To achieve similar goal as git clone --depth=100, but on the server side. I had partial success with doing shallow clone and then converting to bare repo while removing the shallow flag from .git/config. But i didn't liked that solution and wasn't really sure what consequences in terms of data integrity and forward compatibility with newer git versions might be.

To tell you more about my USE CASE:

I want to create free opensource sofware similar to dropbox, but based on git. My idea is following:

1.) Automaticaly pull/commit/push changed files to/from several laptops to single git server (and forcefully resolve all conflicts, this will work unless you plan to use it for software development) 2.) On central server maintain tags indicating latest commits synchronized to individual laptops. 3.) On server delete old commits that are no longer needed by any laptop to sync their worktree. Once synced, delete these commits on laptops as well. (optionaly leaving eg. 1 month or 1GB of old commits in case you might need to rollback. Possibly keep the history only on the server, while deleting it from clients)

This way computers can stay in sync forever without running out of disk space, because old commits are removed. Eg. If i accidentaly add some very big file to synced folder and then delete it, it will eventualy get deleted, once everybody gets in sync again.

I am aware that this is not something which git was designed for, but to me it seems like it should be more than doable. Do you think, any of you can give me some hints on how to approach this problem please?

These are some projects which inspired me to explore this route:

https://github.com/presslabs/gitfs https://www.syncany.org/ https://www.cis.upenn.edu/~bcpierce/unison/ https://etckeeper.branchable.com/