[PATCH 2/4] This exports the update() function from builtin-add.c as

@@ -107,7 +107,7 @@ static void update_callback(struct diff_queue_struct *q,
 	}
 }
 
-static void update(int verbose, const char *prefix, const char **files)
+void add_files_to_cache(int verbose, const char *prefix, const char **files)
 {
 	struct rev_info rev;
 	init_revisions(&rev, prefix);

@@ -116,8 +116,6 @@ static void update(int verbose, const char *prefix, const char **files)
 	rev.diffopt.output_format = DIFF_FORMAT_CALLBACK;
 	rev.diffopt.format_callback = update_callback;
 	rev.diffopt.format_callback_data = &verbose;
-	if (read_cache() < 0)
-		die("index file corrupt");
 	run_diff_files(&rev, 0);
 }
 

@@ -218,7 +216,9 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 	}
 
 	if (take_worktree_changes) {
-		update(verbose, prefix, argv + i);
+		if (read_cache() < 0)
+			die("index file corrupt");
+		add_files_to_cache(verbose, prefix, argv + i);
 		goto finish;
 	}
 

@@ -130,6 +130,8 @@ extern struct commit_list *get_shallow_commits(struct object_array *heads,
 int in_merge_bases(struct commit *, struct commit **, int);
 
 extern int interactive_add(void);
+extern void add_files_to_cache(int verbose,
+			       const char *prefix, const char **files);
 extern int rerere(void);
 
 #endif /* COMMIT_H */