trailer: support multiline title

trailer: support multiline title

We currently ignore the first line passed to `git interpret-trailers`,
when looking for the beginning of the trailers.

Unfortunately this does not work well when a commit is created with a
line break in the title, using for example the following command:

git commit -m 'place of
code: change we made'

That's why instead of ignoring only the first line, it is better to
ignore the first paragraph.

Signed-off-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>