dir: add {new,add}_untracked_cache()

Factor out code into new_untracked_cache() and
add_untracked_cache(), which will be used
in later commits.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/update-index.c

@@ -1123,16 +1123,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			if (untracked_cache == UC_TEST)
 				return 0;
 		}
-		if (!the_index.untracked) {
-			struct untracked_cache *uc = xcalloc(1, sizeof(*uc));
-			strbuf_init(&uc->ident, 100);
-			uc->exclude_per_dir = ".gitignore";
-			/* should be the same flags used by git-status */
-			uc->dir_flags = DIR_SHOW_OTHER_DIRECTORIES | DIR_HIDE_EMPTY_DIRECTORIES;
-			the_index.untracked = uc;
-		}
-		add_untracked_ident(the_index.untracked);
-		the_index.cache_changed |= UNTRACKED_CHANGED;
+		add_untracked_cache(&the_index);
 		report(_("Untracked cache enabled for '%s'"), get_git_work_tree());
 	} else if (untracked_cache == UC_DISABLE) {
 		if (the_index.untracked) {

dir.c

@@ -1938,6 +1938,24 @@ void add_untracked_ident(struct untracked_cache *uc)
 	strbuf_addch(&uc->ident, 0);
 }
 
+static void new_untracked_cache(struct index_state *istate)
+{
+	struct untracked_cache *uc = xcalloc(1, sizeof(*uc));
+	strbuf_init(&uc->ident, 100);
+	uc->exclude_per_dir = ".gitignore";
+	/* should be the same flags used by git-status */
+	uc->dir_flags = DIR_SHOW_OTHER_DIRECTORIES | DIR_HIDE_EMPTY_DIRECTORIES;
+	istate->untracked = uc;
+}
+
+void add_untracked_cache(struct index_state *istate)
+{
+	if (!istate->untracked) {
+		new_untracked_cache(istate);
+	add_untracked_ident(istate->untracked);
+	istate->cache_changed |= UNTRACKED_CHANGED;
+}
+
 static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *dir,
 						      int base_len,
 						      const struct pathspec *pathspec)

dir.h

@@ -308,4 +308,5 @@ void free_untracked_cache(struct untracked_cache *);
 struct untracked_cache *read_untracked_extension(const void *data, unsigned long sz);
 void write_untracked_extension(struct strbuf *out, struct untracked_cache *untracked);
 void add_untracked_ident(struct untracked_cache *);
+void add_untracked_cache(struct index_state *istate);
 #endif