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

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>