test-dump-untracked-cache: don't modify the untracked cache

To correctly perform its testing function,
test-dump-untracked-cache should not change the state of the
untracked cache in the index.

As a previous patch makes read_index_from() change the state of
the untracked cache and as test-dump-untracked-cache indirectly
calls this function, we need a mechanism to prevent
read_index_from() from changing the untracked cache state when
it's called from test-dump-untracked-cache.

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

config.c

@@ -1599,6 +1599,10 @@ int git_config_get_untracked_cache(void)
 	int val = -1;
 	const char *v;
 
+	/* Hack for test programs like test-dump-untracked-cache */
+	if (ignore_untracked_cache_config)
+		return -1;
+
 	if (!git_config_get_maybe_bool("core.untrackedcache", &val))
 		return val;