Remove old commit information from a git repository to save space

I think, you can shrink your history following this answer:

How to delete a specific revision of a github gist?

Decide on which points in history, you want to keep.

pick <hash1> <commit message>
pick <hash2> <commit message>
pick <hash3> <commit message>   <- keep
pick <hash4> <commit message>
pick <hash5> <commit message>
pick <hash6> <commit message>   <- keep
pick <hash7> <commit message>
pick <hash8> <commit message>
pick <hash9> <commit message>
pick <hash10> <commit message>  <- keep

Then, leave the first after each "keep" as "pick" and mark the others as "squash".

pick   <hash1> <commit message>
squash <hash2> <commit message>
squash <hash3> <commit message>   <- keep
pick   <hash4> <commit message>
squash <hash5> <commit message>
squash <hash6> <commit message>   <- keep
pick   <hash7> <commit message>
squash <hash8> <commit message>
squash <hash9> <commit message>
squash <hash10> <commit message>  <- keep

Then, run the rebase by saving and quitting the editor. At each "keep" point, the message editor will pop up for a combined commit message ranging from the previous "pick" up to the "keep" commit. You can then either just keep the last message or in fact combine those to document the original history without keeping all intermediate states.

After that rebase, the intermediate file data will still be in the repository but now unreferenced. git gc will now indeed get you rid of that data.

You could always just delete .git and do a fresh git init with one initial commit. This will, of course, remove all commit history.