Merge branch 'ds/core-untracked-cache-config'

Setting core.untrackedCache to true failed to add the untracked
cache extension to the index.

* ds/core-untracked-cache-config:
  dir: force untracked cache with core.untrackedCache

Author: gitster

dir.c

@@ -2936,7 +2936,9 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
 
 		if (force_untracked_cache < 0)
 			force_untracked_cache =
-				git_env_bool("GIT_FORCE_UNTRACKED_CACHE", 0);
+				git_env_bool("GIT_FORCE_UNTRACKED_CACHE", -1);
+		if (force_untracked_cache < 0)
+			force_untracked_cache = (istate->repo->settings.core_untracked_cache == UNTRACKED_CACHE_WRITE);
 		if (force_untracked_cache &&
 			dir->untracked == istate->untracked &&
 		    (dir->untracked->dir_opened ||