Move git utilities into a dedicated crate (#15564)

- create `codex-git-utils` and move the shared git helpers into it with
file moves preserved for diff readability
- move the `GitInfo` helpers out of `core` so stacked rollout work can
depend on the shared crate without carrying its own git info module

---------

Co-authored-by: Ahmed Ibrahim <219906144+aibrahim-oai@users.noreply.github.com>
Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/src/git_info_tests.rs

@@ -1,9 +1,15 @@
-use super::*;
-
+use codex_git_utils::GitInfo;
+use codex_git_utils::GitSha;
+use codex_git_utils::collect_git_info;
+use codex_git_utils::get_has_changes;
+use codex_git_utils::git_diff_to_remote;
+use codex_git_utils::recent_commits;
+use codex_git_utils::resolve_root_git_project_for_trust;
 use core_test_support::skip_if_sandbox;
 use std::fs;
 use std::path::PathBuf;
 use tempfile::TempDir;
+use tokio::process::Command;
 
 // Helper function to create a test git repository
 async fn create_test_git_repo(temp_dir: &TempDir) -> PathBuf {
@@ -191,7 +197,7 @@ async fn test_collect_git_info_git_repository() {
 
     // Should have commit hash
     assert!(git_info.commit_hash.is_some());
-    let commit_hash = git_info.commit_hash.unwrap();
+    let commit_hash = git_info.commit_hash.unwrap().0;
     assert_eq!(commit_hash.len(), 40); // SHA-1 hash should be 40 characters
     assert!(commit_hash.chars().all(|c| c.is_ascii_hexdigit()));
 
@@ -558,7 +564,7 @@ async fn test_get_git_working_tree_state_unpushed_commit() {
 #[test]
 fn test_git_info_serialization() {
     let git_info = GitInfo {
-        commit_hash: Some("abc123def456".to_string()),
+        commit_hash: Some(GitSha::new("abc123def456")),
         branch: Some("main".to_string()),
         repository_url: Some("https://github.com/example/repo.git".to_string()),
     };