t6300: refactor %(trailers) tests

t6300: refactor %(trailers) tests

We currently have one test for %(trailers) in `git-for-each-ref(1)`,
through "%(contents:trailers)". In preparation for more, let's add a few
things:

  - Move the commit creation step to its own test so that it can be
  re-used.

  - Add a non-trailer to the commit's trailers to test that non-trailers
  aren't shown using "%(trailers:only)".

  - Add a multi-line trailer to ensure that trailers are unfolded
  correctly using "%(trailers:unfold)".

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>