tests: don't lose "git" exit codes in "! ( git ... | grep )"

tests: don't lose "git" exit codes in "! ( git ... | grep )"

Change tests that would lose the "git" exit code via a negation
pattern to:

- In the case of "t0055-beyond-symlinks.sh" compare against the
  expected output instead.

  We could use the same pattern as in the "t3700-add.sh" below, doing
  so would have the advantage that if we added an earlier test we
  wouldn't need to adjust the "expect" output.

  But as "t0055-beyond-symlinks.sh" is a small and focused test (less
  than 40 lines in total) let's use "test_cmp" instead.

- For "t3700-add.sh" use "sed -n" to print the expected "bad" part,
  and use "test_must_be_empty" to assert that it's not there. If we used
  "grep" we'd get a non-zero exit code.

  We could use "test_expect_code 1 grep", but this is more consistent
  with existing patterns in the test suite.

  We can also remove a repeated invocation of "git ls-files" for the
  last test that's being modified in that file, and search the
  existing "files" output instead.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>