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.  The
tests [1] checking that 'git rebase --quit' and autostash work
together as expected after such a failure then run '! grep ...' to
ensure that the dirty contents of the 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, and thus it shouldn't
exist after the subsequent 'git rebase --quit' either.  Consequently,
this 'grep' doesn't fail as expected, i.e. because it can't find the
dirty content, but instead it fails, because it can't open the file.

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

Previously 2745817028 (t3420-rebase-autostash: don't try to grep
non-existing files, 2018-08-22) fixed a couple of similar issues; this
one was added later in 9b2df3e8d0 (rebase: save autostash entry into
stash reflog on --quit, 2020-04-28).

[1] This patch modifies only a single test, but that test is run
    several times with different strategies ('--apply', '--merge', and
    '--interactive'), hence the plural "tests".

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