git: Use the CLI for loading commit SHAs and details (#29351)

Since #28065 merged we've seen deadlocks inside iconv when opening Zed
in a repository containing many submodules. These calls to iconv happen
inside libgit2, in our implementations of the methods `head_sha`,
`merge_head_shas`, and `show` on `RealGitRepository`. This PR moves
those methods to use the git CLI instead, sidestepping the issue. For
the sake of efficiency, a new `revparse_batch` method is added that uses
`git cat-file` to resolve several ref names in one invocation. I
originally intended to make `show` operate in batch mode as well (or
instead), but I can't see a good way to do that with the git CLI; `git
show` always bails on the first ref that it can't resolve, and
`for-each-ref` doesn't support symbolic refs like `HEAD`.

Separately, I removed the calls to `show` in `MergeDetails::load`, going
back to only loading the SHAs of the various merge heads. Loading full
commit details was intended to support the inlays feature that ended up
being cut from #28065, and we can add it back in when we need it.

Release Notes:

- N/A

crates/agent/src/thread.rs

@@ -1964,7 +1964,7 @@ impl Thread {
                             };
 
                             let remote_url = backend.remote_url("origin");
-                            let head_sha = backend.head_sha();
+                            let head_sha = backend.head_sha().await;
                             let diff = backend.diff(DiffType::HeadToWorktree).await.ok();
 
                             GitState {