t1092: fix buggy sparse "blame" test

t1092: fix buggy sparse "blame" test

This test wants to verify that `git blame` errors out when asked to
blame a file _not_ in the sparse checkout. However, the very first file
it asks to blame _is_ present in the checkout, thus `test_must_fail git
blame $file` gives an unexpected result (the "blame" succeeds). This
problem went unnoticed because the test invokes `test_must_fail git
blame $file` in loop but forgets to break out of the loop early upon
failure, thus the failure gets swallowed.

Fix the test by having it not ask to blame a file present in the sparse
checkout, and instead only blame files not present, as intended. While
at it, also add the missing `|| return 1` which allowed this bug to go
unnoticed.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>