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/tui/src/chatwidget/tests.rs

@@ -51,6 +51,7 @@ use codex_core::plugins::OPENAI_CURATED_MARKETPLACE_NAME;
 use codex_core::skills::model::SkillMetadata;
 use codex_features::FEATURES;
 use codex_features::Feature;
+use codex_git_utils::CommitLogEntry;
 use codex_otel::RuntimeMetricsSummary;
 use codex_otel::SessionTelemetry;
 use codex_protocol::ThreadId;
@@ -6725,12 +6726,12 @@ async fn review_commit_picker_shows_subjects_without_timestamps() {
 
     // Show commit picker with synthetic entries.
     let entries = vec![
-        codex_core::git_info::CommitLogEntry {
+        CommitLogEntry {
             sha: "1111111deadbeef".to_string(),
             timestamp: 0,
             subject: "Add new feature X".to_string(),
         },
-        codex_core::git_info::CommitLogEntry {
+        CommitLogEntry {
             sha: "2222222cafebabe".to_string(),
             timestamp: 0,
             subject: "Fix bug Y".to_string(),