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/app-server/src/codex_message_processor.rs

@@ -213,7 +213,6 @@ use codex_core::find_archived_thread_path_by_id_str;
 use codex_core::find_thread_name_by_id;
 use codex_core::find_thread_names_by_ids;
 use codex_core::find_thread_path_by_id_str;
-use codex_core::git_info::git_diff_to_remote;
 use codex_core::mcp::auth::discover_supported_scopes;
 use codex_core::mcp::auth::resolve_oauth_scopes;
 use codex_core::mcp::collect_mcp_snapshot;
@@ -243,6 +242,7 @@ use codex_features::FEATURES;
 use codex_features::Feature;
 use codex_features::Stage;
 use codex_feedback::CodexFeedback;
+use codex_git_utils::git_diff_to_remote;
 use codex_login::ServerOptions as LoginServerOptions;
 use codex_login::ShutdownHandle;
 use codex_login::auth::login_with_chatgpt_auth_tokens;
@@ -8224,7 +8224,7 @@ fn extract_conversation_summary(
 
 fn map_git_info(git_info: &CoreGitInfo) -> ConversationGitInfo {
     ConversationGitInfo {
-        sha: git_info.commit_hash.clone(),
+        sha: git_info.commit_hash.as_ref().map(|sha| sha.0.clone()),
         branch: git_info.branch.clone(),
         origin_url: git_info.repository_url.clone(),
     }