Add option to ship vendored libgit2 dependency

Basically, just add a feature `vendored-libgit2 = ["git2/vendored-libgit2"]` ([just like `eza` has](https://github.com/eza-community/eza/blob/99562e3a4f0e3cf76444b0322772970d4773ce71/Cargo.toml#L124)) to allow running `bat` without having `libgit2` installed on the system.