t3903-stash: don't try to grep non-existing file

The test 'store updates stash ref and reflog' in 't3903-stash.sh' creates a stash from a new file, runs 'git reset --hard' to throw away any modifications to the work tree, and then runs '! grep' to ensure that the staged contents are gone. Since the file didn't exist before, it shouldn't exist after 'git reset' either. Consequently, this 'grep' doesn't fail as expected, because it can't find the staged content, but it fails because it can't open the file. Tighten this check by using 'test_path_is_missing' instead, thereby avoiding an unexpected error from 'grep' as well. Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>