untracked cache: use git_env_bool() not getenv() for customization

GIT_DISABLE_UNTRACKED_CACHE and GIT_TEST_UNTRACKED_CACHE are only
sensed for their presense by using getenv(); use git_env_bool()
instead so that GIT_DISABLE_UNTRACKED_CACHE=false would work as
naïvely expected.

Also rename GIT_TEST_UNTRACKED_CACHE to GIT_FORCE_UNTRACKED_CACHE
to express what it does more honestly.  Forcing its use may be one
useful thing to do while testing the feature, but testing does not
have to be the only use of the knob.

While at it, avoid repeated calls to git_env_bool() by capturing the
return value from the first call in a static variable.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

dir.c

@@ -2164,8 +2164,13 @@ static struct untracked_cache_dir *validate_untracked_cache(struct dir_struct *d
 						      const struct pathspec *pathspec)
 {
 	struct untracked_cache_dir *root;
+	static int untracked_cache_disabled = -1;
 
-	if (!dir->untracked || getenv("GIT_DISABLE_UNTRACKED_CACHE"))
+	if (!dir->untracked)
+		return NULL;
+	if (untracked_cache_disabled < 0)
+		untracked_cache_disabled = git_env_bool("GIT_DISABLE_UNTRACKED_CACHE", 0);
+	if (untracked_cache_disabled)
 		return NULL;
 
 	/*
@@ -2287,7 +2292,12 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
 	}
 
 	if (dir->untracked) {
+		static int force_untracked_cache = -1;
 		static struct trace_key trace_untracked_stats = TRACE_KEY_INIT(UNTRACKED_STATS);
+
+		if (force_untracked_cache < 0)
+			force_untracked_cache =
+				git_env_bool("GIT_FORCE_UNTRACKED_CACHE", 0);
 		trace_printf_key(&trace_untracked_stats,
 				 "node creation: %u\n"
 				 "gitignore invalidation: %u\n"
@@ -2297,7 +2307,7 @@ int read_directory(struct dir_struct *dir, struct index_state *istate,
 				 dir->untracked->gitignore_invalidated,
 				 dir->untracked->dir_invalidated,
 				 dir->untracked->dir_opened);
-		if (getenv("GIT_TEST_UNTRACKED_CACHE") &&
+		if (force_untracked_cache &&
 			dir->untracked == istate->untracked &&
 		    (dir->untracked->dir_opened ||
 		     dir->untracked->gitignore_invalidated ||

t/t7063-status-untracked-cache.sh

@@ -14,8 +14,8 @@ test_description='test untracked cache'
 # See <20160803174522.5571-1-pclouds@gmail.com> if you want to know
 # more.
 
-GIT_TEST_UNTRACKED_CACHE=true
-export GIT_TEST_UNTRACKED_CACHE
+GIT_FORCE_UNTRACKED_CACHE=true
+export GIT_FORCE_UNTRACKED_CACHE
 
 sync_mtime () {
 	find . -type d -ls >/dev/null