t3903: avoid applying onto dirty index

t3903: avoid applying onto dirty index

One of the tests in t3903 wants to make sure that applying a
stash that touches only "file" can still happen even if there
are working tree changes to "other-file". To do so, it adds
"other-file" to the index (since otherwise it is an
untracked file, voiding the purpose of the test).

But as we are about to refactor the dirty-index handling,
and as this test does not actually care about having a dirty
index (only a dirty working tree), let's bump the tracking
of "other-file" into the setup phase, so we can have _just_
a dirty working tree here.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>