diff: implement --no-optional-locks

When used with `autoRefreshIndex`, `git diff` may update the index in
the background for similar performance reasons to `git-status`.

This commit implements the `--no-optional-locks` option for `git diff`,
which allows scripts to bypass this behavior.

Signed-off-by: Benjamin Woodruff <benjamin.woodruff@vercel.com>

Author: bgw

Diff for: Documentation/config/diff.adoc

@@ -6,7 +6,9 @@
 	contents in the work tree match the contents in the
 	index.  This option defaults to `true`.  Note that this
 	affects only `git diff` Porcelain, and not lower level
-	`diff` commands such as `git diff-files`.
+	`diff` commands such as `git diff-files`. If
+	`--no-optional-locks` is set (see linkgit:git[1] for
+	details), the index file is not updated.
 
 `diff.dirstat`::
 ifdef::git-diff[]

Diff for: Documentation/git.adoc

@@ -190,7 +190,8 @@ If you just want to run git as if it was started in `<path>` then use
 --no-optional-locks::
 	Do not perform optional operations that require locks. This is
 	equivalent to setting the `GIT_OPTIONAL_LOCKS` to `0`. This
-	functionality is implemented for `git status` and `git describe`.
+	functionality is implemented for `git status`, `git describe`,
+	and `git diff`.
 
 --no-advice::
 	Disable all advice hints from being printed.

Diff for: builtin/diff.c

@@ -9,6 +9,7 @@
 
 #include "builtin.h"
 #include "config.h"
+#include "environment.h"
 #include "ewah/ewok.h"
 #include "lockfile.h"
 #include "color.h"
@@ -239,6 +240,9 @@ static void refresh_index_quietly(void)
 	struct lock_file lock_file = LOCK_INIT;
 	int fd;
 
+	if (!use_optional_locks())
+		return;
+
 	fd = repo_hold_locked_index(the_repository, &lock_file, 0);
 	if (fd < 0)
 		return;

Diff for: t/meson.build

@@ -500,6 +500,7 @@ integration_tests = [
   't4067-diff-partial-clone.sh',
   't4068-diff-symmetric-merge-base.sh',
   't4069-remerge-diff.sh',
+  't4070-diff-auto-refresh-index.sh',
   't4100-apply-stat.sh',
   't4101-apply-nonl.sh',
   't4102-apply-rename.sh',

Diff for: t/t4070-diff-auto-refresh-index.sh

@@ -0,0 +1,46 @@
+#!/bin/sh
+#
+# Copyright (c) 2025 Benjamin Woodruff
+#
+
+test_description='diff.autoRefreshIndex config option'
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-diff.sh
+
+test_expect_success 'index is updated when autoRefreshIndex is true' '
+	>tracked &&
+	git add tracked &&
+
+	# stat() must change (but not file contents) to trigger an index update
+	test_set_magic_mtime tracked &&
+
+	# check the mtime of .git/index does not change without autoRefreshIndex
+	test_set_magic_mtime .git/index &&
+	git config diff.autoRefreshIndex false &&
+	git diff &&
+	test_is_magic_mtime .git/index &&
+
+	# but it does change when autoRefreshIndex is true (the default)
+	git config diff.autoRefreshIndex true &&
+	git diff &&
+	! test_is_magic_mtime .git/index
+'
+
+test_expect_success '--no-optional-locks overrides autoRefreshIndex' '
+	>tracked &&
+	git add tracked &&
+	test_set_magic_mtime tracked &&
+
+	# `--no-optional-locks` overrides `autoRefreshIndex`
+	test_set_magic_mtime .git/index &&
+	git config diff.autoRefreshIndex true &&
+	git --no-optional-locks diff &&
+
+	# sanity check that without `--no-optional-locks` it still updates
+	test_is_magic_mtime .git/index &&
+	git diff &&
+	! test_is_magic_mtime .git/index
+'
+
+test_done