for-each-ref: add "upstream" format field

for-each-ref: add "upstream" format field

The logic for determining the upstream ref of a branch is
somewhat complex to perform in a shell script. This patch
provides a plumbing mechanism for scripts to access the C
logic used internally by git-status, git-branch, etc.

For example:

  $ git for-each-ref \
       --format='%(refname:short) %(upstream:short)' \
       refs/heads/
  master origin/master

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>