t3420-rebase-autostash: don't try to grep non-existing files

t3420-rebase-autostash: don't try to grep non-existing files

Several tests in 't3420-rebase-autostash.sh' start various rebase
processes that are expected to fail because of merge conflicts.  These
tests then run '! grep' to ensure that the autostash feature did its
job, and the dirty contents of a file is gone.  However, due to the
test repo's history and the choice of upstream branch that file
shouldn't exist in the conflicted state at all.  Consequently, this
'grep' doesn't fail as expected, because it can't find the dirty
content, but it fails because it can't open the file.

Tighten this check by using 'test_path_is_missing' instead, thereby
avoiding unexpected errors from 'grep' as well.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>