thodg/libgit2

diff --git a/src/diff.c b/src/diff.c
index 12d34a1..c1adcc6 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -664,8 +664,10 @@ int git_diff__oid_for_entry(
 		updated_entry.mode = mode;
 		git_oid_cpy(&updated_entry.id, out);
 
-		if (!(error = git_repository_index__weakptr(&idx, diff->repo)))
+		if (!(error = git_repository_index__weakptr(&idx, diff->repo))) {
 			error = git_index_add(idx, &updated_entry);
+			diff->index_updated = true;
+		}
  	}
 
 	git_buf_free(&full_path);
@@ -1360,7 +1362,7 @@ int git_diff_index_to_workdir(
 			&b, repo, index, NULL, GIT_ITERATOR_DONT_AUTOEXPAND, pfx, pfx)
 	);
 
-	if (!error && DIFF_FLAG_IS_SET(*diff, GIT_DIFF_UPDATE_INDEX))
+	if (!error && DIFF_FLAG_IS_SET(*diff, GIT_DIFF_UPDATE_INDEX) && (*diff)->index_updated)
 		error = git_index_write(index);
 
 	return error;
diff --git a/src/diff.h b/src/diff.h
index a202a08..2dfc2c6 100644
--- a/src/diff.h
+++ b/src/diff.h
@@ -64,6 +64,7 @@ struct git_diff {
 	git_iterator_type_t new_src;
 	uint32_t diffcaps;
 	git_diff_perfdata perf;
+	bool index_updated;
 
 	int (*strcomp)(const char *, const char *);
 	int (*strncomp)(const char *, const char *, size_t);
diff --git a/tests/diff/workdir.c b/tests/diff/workdir.c
index 13de6a9..34bad48 100644
--- a/tests/diff/workdir.c
+++ b/tests/diff/workdir.c
@@ -1544,19 +1544,21 @@ void test_diff_workdir__with_stale_index(void)
 
 static int touch_file(void *payload, git_buf *path)
 {
-	int fd;
-	char b;
+	struct stat st;
+	struct timeval times[2];
 
 	GIT_UNUSED(payload);
 	if (git_path_isdir(path->ptr))
 		return 0;
 
-	cl_assert((fd = p_open(path->ptr, O_RDWR)) >= 0);
-	cl_assert_equal_i(1, p_read(fd, &b, 1));
-	cl_must_pass(p_lseek(fd, 0, SEEK_SET));
-	cl_must_pass(p_write(fd, &b, 1));
-	cl_must_pass(p_close(fd));
+	cl_must_pass(p_stat(path->ptr, &st));
+
+	times[0].tv_sec = st.st_mtime + 3;
+	times[0].tv_usec = 0;
+	times[1].tv_sec = st.st_mtime + 3;
+	times[1].tv_usec = 0;
 
+	cl_must_pass(p_utimes(path->ptr, times));
 	return 0;
 }
 
@@ -1783,3 +1785,63 @@ void test_diff_workdir__to_index_conflicted(void) {
 	git_index_free(index);
 	git_tree_free(a);
 }
+
+void test_diff_workdir__only_writes_index_when_necessary(void)
+{
+	git_index *index;
+	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
+	git_diff *diff = NULL;
+	git_diff_perfdata perf = GIT_DIFF_PERFDATA_INIT;
+	git_reference *head;
+	git_object *head_object;
+	git_oid initial, first, second;
+	git_buf path = GIT_BUF_INIT;
+	struct stat st;
+	struct timeval times[2];
+
+	opts.flags |= GIT_DIFF_INCLUDE_UNTRACKED | GIT_DIFF_UPDATE_INDEX;
+
+	g_repo = cl_git_sandbox_init("status");
+
+	cl_git_pass(git_repository_index(&index, g_repo));
+	cl_git_pass(git_repository_head(&head, g_repo));
+	cl_git_pass(git_reference_peel(&head_object, head, GIT_OBJ_COMMIT));
+
+	cl_git_pass(git_reset(g_repo, head_object, GIT_RESET_HARD, NULL));
+
+	git_oid_cpy(&initial, git_index_checksum(index));
+
+	/* update the index timestamp to avoid raciness */
+	cl_must_pass(p_stat("status/.git/index", &st));
+
+	times[0].tv_sec = st.st_mtime + 5;
+	times[0].tv_usec = 0;
+	times[1].tv_sec = st.st_mtime + 5;
+	times[1].tv_usec = 0;
+
+	cl_must_pass(p_utimes("status/.git/index", times));
+
+	/* ensure diff doesn't touch the index */
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+	git_diff_free(diff);
+
+	git_oid_cpy(&first, git_index_checksum(index));
+	cl_assert(!git_oid_equal(&initial, &first));
+
+	/* touch all the files so stat times are different */
+	cl_git_pass(git_buf_sets(&path, "status"));
+	cl_git_pass(git_path_direach(&path, 0, touch_file, NULL));
+
+	cl_git_pass(git_diff_index_to_workdir(&diff, g_repo, NULL, &opts));
+	git_diff_free(diff);
+
+	/* ensure the second diff did update the index */
+	git_oid_cpy(&second, git_index_checksum(index));
+	cl_assert(!git_oid_equal(&first, &second));
+
+	git_buf_free(&path);
+	git_object_free(head_object);
+	git_reference_free(head);
+	git_index_free(index);
+}
+