t4205: refactor %(trailers) tests

t4205: refactor %(trailers) tests

We currently have one test for %(trailers). In preparation
for more, let's refactor a few bits:

  - move the commit creation to its own setup step so it can
    be reused by multiple tests

  - add a trailer with whitespace continuation (to confirm
    that it is left untouched)

  - fix the sample text which claims the placeholder is %bT.
    This was switched long ago to %(trailers)

  - replace one "cat" with an "echo" when generating the
    expected output. This saves a process (and sets a better
    pattern for future tests to follow).

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