t7519: populate untracked cache before test

t7519: populate untracked cache before test

In its current state, the t7519 test dealing with untracked cache
assumes that "git update-index --untracked-cache" will *populate* the
untracked cache. This is not correct - it will only add an empty
untracked cache structure to the index.

If we're going to compare two git status runs with something
interesting happening in-between, we need to ensure that the index is
in a stable/steady state *before* that first run.

Achieve this by adding another prior "git status" run.

At this stage this change does nothing, because there is a bug,
addressed in the next patch, whereby once the empty untracked cache
structure is added by the update-index invocation, the untracked cache
gets updated in every subsequent "git status" call, but the index with
these updates does not get written down.

That bug actually invalidates this entire test case - but we're fixing
that next.

Signed-off-by: Tao Klerks <tao@klerks.biz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>