The Amendment class calls `git show` twice, once to extract the commit
title, and a second time to extract the trailers. This can be combined
in a single command, which is more efficient. Do so.
While at it, centralize initialization of self._trailers in the
Commit.__init__() function.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>