trailer: have function to describe trailer layout

trailer: have function to describe trailer layout

Create a function that, taking a string, describes the position of its
trailer block (if available) and the contents thereof, and make trailer
use it. This makes it easier for other Git components, in the future, to
interpret trailer blocks in the same way as trailer.

In a subsequent patch, another component will be made to use this.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>