All of the information needed to find the @{upstream} of a
branch is included in the branch struct, but callers have to
navigate a series of possible-NULL values to get there.
Let's wrap that logic up in an easy-to-read helper.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>