t1300: put git invocations inside test function

t1300: put git invocations inside test function

This is a very old script, and did a lot of:

  echo whatever >expect
  git config foo bar
  test_expect_success 'cmp .git/config expect'

which meant that we didn't actually check that the call to
git-config succeeded. Fix this, and while we're at it,
modernize the style to use test_cmp.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>