I don't know how far he intends to take it, but the author of gitoxide (which is just a library, like libgit2) uses it to power a git CLI called gix. As of October it looks like it can do fetches and clones: https://github.com/Byron/gitoxide/discussions/623 .

Since the git CLI is subcommand-based, it wouldn't be too hard to provide replacements for individual git subcommands one at a time. If you replaced git clone, push, pull, commit, checkout, and rebase, that would suffice for 95% of git invocations.