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

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>