feat: add support for /diff command (#1389)

Adds support for a `/diff` command comparable to the one available in
the TypeScript CLI.

<img width="1103" alt="Screenshot 2025-06-26 at 12 31 33 PM"
src="https://github.com/user-attachments/assets/5dc646ca-301f-41ff-92a7-595c68db64b6"
/>

While here, changed the `SlashCommand` enum so the declared variant
order is the order the commands appear in the popup menu. This way,
`/toggle-mouse-mode` is listed last, as it is the least likely to be
used.

Fixes https://github.com/openai/codex/issues/1253.

codex-rs/tui/src/get_git_diff.rs

@@ -0,0 +1,114 @@
+//! Utility to compute the current Git diff for the working directory.
+//!
+//! The implementation mirrors the behaviour of the TypeScript version in
+//! `codex-cli`: it returns the diff for tracked changes as well as any
+//! untracked files. When the current directory is not inside a Git
+//! repository, the function returns `Ok((false, String::new()))`.
+
+use std::io;
+use std::path::Path;
+use std::process::Command;
+use std::process::Stdio;
+
+/// Return value of [`get_git_diff`].
+///
+/// * `bool` – Whether the current working directory is inside a Git repo.
+/// * `String` – The concatenated diff (may be empty).
+pub(crate) fn get_git_diff() -> io::Result<(bool, String)> {
+    // First check if we are inside a Git repository.
+    if !inside_git_repo()? {
+        return Ok((false, String::new()));
+    }
+
+    // 1. Diff for tracked files.
+    let tracked_diff = run_git_capture_diff(&["diff", "--color"])?;
+
+    // 2. Determine untracked files.
+    let untracked_output = run_git_capture_stdout(&["ls-files", "--others", "--exclude-standard"])?;
+
+    let mut untracked_diff = String::new();
+    let null_device: &Path = if cfg!(windows) {
+        Path::new("NUL")
+    } else {
+        Path::new("/dev/null")
+    };
+
+    for file in untracked_output
+        .split('\n')
+        .map(str::trim)
+        .filter(|s| !s.is_empty())
+    {
+        // Use `git diff --no-index` to generate a diff against the null device.
+        let args = [
+            "diff",
+            "--color",
+            "--no-index",
+            "--",
+            null_device.to_str().unwrap_or("/dev/null"),
+            file,
+        ];
+
+        match run_git_capture_diff(&args) {
+            Ok(diff) => untracked_diff.push_str(&diff),
+            // If the file disappeared between ls-files and diff we ignore the error.
+            Err(err) if err.kind() == io::ErrorKind::NotFound => {}
+            Err(err) => return Err(err),
+        }
+    }
+
+    Ok((true, format!("{}{}", tracked_diff, untracked_diff)))
+}
+
+/// Helper that executes `git` with the given `args` and returns `stdout` as a
+/// UTF-8 string. Any non-zero exit status is considered an *error*.
+fn run_git_capture_stdout(args: &[&str]) -> io::Result<String> {
+    let output = Command::new("git")
+        .args(args)
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .output()?;
+
+    if output.status.success() {
+        Ok(String::from_utf8_lossy(&output.stdout).into_owned())
+    } else {
+        Err(io::Error::other(format!(
+            "git {:?} failed with status {}",
+            args, output.status
+        )))
+    }
+}
+
+/// Like [`run_git_capture_stdout`] but treats exit status 1 as success and
+/// returns stdout. Git returns 1 for diffs when differences are present.
+fn run_git_capture_diff(args: &[&str]) -> io::Result<String> {
+    let output = Command::new("git")
+        .args(args)
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .output()?;
+
+    if output.status.success() || output.status.code() == Some(1) {
+        Ok(String::from_utf8_lossy(&output.stdout).into_owned())
+    } else {
+        Err(io::Error::other(format!(
+            "git {:?} failed with status {}",
+            args, output.status
+        )))
+    }
+}
+
+/// Determine if the current directory is inside a Git repository.
+fn inside_git_repo() -> io::Result<bool> {
+    let status = Command::new("git")
+        .args(["rev-parse", "--is-inside-work-tree"])
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .status();
+
+    match status {
+        Ok(s) if s.success() => Ok(true),
+        Ok(_) => Ok(false),
+        Err(e) if e.kind() == io::ErrorKind::NotFound => Ok(false), // git not installed
+        Err(e) => Err(e),
+    }
+}