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/lib.rs

@@ -41,7 +41,8 @@ mod exec_policy;
 pub mod external_agent_config;
 pub mod file_watcher;
 mod flags;
-pub mod git_info;
+#[cfg(test)]
+mod git_info_tests;
 mod guardian;
 mod hook_runtime;
 pub mod instructions;