dir: force untracked cache with core.untrackedCache

The GIT_FORCE_UNTRACKED_CACHE environment variable write the untracked
cache more frequently than the core.untrackedCache config variable. This
is due to how read_directory() handles the creation of an untracked
cache. The old mechanism required using something like 'git update-index
--untracked-cache' before the index would actually contain an untracked
cache. This was noted as a performance problem on macOS in the past, and
this is the resolution for that issue.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

dir.c

@@ -3053,7 +3053,9 @@ 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 ||