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.

that would fit how git itself evolved. A fair amount of code was in perl and stayed that way for awhile before being written in C.

many of the porcelain commands are or used to be in perl/bash

A question that comes to mind: should all Perl scripts be rewritten in C before introducing Rust into the code base? Practically speaking, this probably might never happen, but perhaps the trade-off in technical debt is worth it.