feat: /diff command to view git diff (#426)

Adds `/diff` command to view git diff

codex-cli/src/utils/get-diff.ts

@@ -0,0 +1,29 @@
+import { execSync } from "node:child_process";
+
+/**
+ * Returns the current Git diff for the working directory. If the current
+ * working directory is not inside a Git repository, `isGitRepo` will be
+ * false and `diff` will be an empty string.
+ */
+export function getGitDiff(): {
+  isGitRepo: boolean;
+  diff: string;
+} {
+  try {
+    // First check whether we are inside a git repository. `rev‑parse` exits
+    // with a non‑zero status code if not.
+    execSync("git rev-parse --is-inside-work-tree", { stdio: "ignore" });
+
+    // If the above call didn’t throw, we are inside a git repo. Retrieve the
+    // diff including color codes so that the overlay can render them.
+    const output = execSync("git diff --color", {
+      encoding: "utf8",
+      maxBuffer: 10 * 1024 * 1024, // 10 MB ought to be enough for now
+    });
+
+    return { isGitRepo: true, diff: output };
+  } catch {
+    // Either git is not installed or we’re not inside a repository.
+    return { isGitRepo: false, diff: "" };
+  }
+}