t5701: modernize style

t5701: modernize style

This test is pretty old and did not follow some of our more
modern best practices. In particular:

  1. It chdir'd all over the place, leaving later tests to
     deal with the fallout. Do our chdirs in subshells
     instead.

  2. It did not use test_must_fail.

  3. It did not use test_line_count.

  4. It checked for the non-existence of a ref by looking in the
     .git/refs directory (since we pack refs during clone
     these days, this will always be succeed, making the
     test useless).

     Note that one call to "-e .git/refs/..." remains,
     because it is checking for the existence of a symbolic
     ref, not a ref itself.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>