Compute git statuses using the bundled git executable, not libgit2 (#12444)

I realized that somehow, the `git` executable is able to compute `git
status` much more quickly than libgit2, so I've switched our git status
logic to use `git`. Follow-up to
https://github.com/zed-industries/zed/pull/12266.

Release Notes:

- Improved the performance of git status updated when working in large
git repositories.

crates/git/src/git.rs

@@ -15,6 +15,7 @@ pub mod blame;
 pub mod commit;
 pub mod diff;
 pub mod repository;
+pub mod status;
 
 lazy_static! {
     pub static ref DOT_GIT: &'static OsStr = OsStr::new(".git");