Migrate to CLI git fetching

Over time, there have been enough issues that crop up with libgit2's fetching, either performance, lack of features, separate network configuration, etc that migrating to the CLI for fetching may be justified. Gitoxide could eventually be an alternative but we then are still playing catch up with each new git feature. The question is whether we should transition away from supporting libgit2 fetching. Potentially related issues - #10280 - #8172 - #10230 - #10583 - #14617 - #16803 - #2078 - #8918 - #15184 - #9167 - #11577 - #12524 - #13624 - #5256 - #11165 - #15481 - #7497 Reported issues: - #15775 - `corrupt_git.rs` tests (https://github.com/rust-lang/cargo/issues/17227#issuecomment-5025688191)