branch: give advice when tracking start-point is missing

branch: give advice when tracking start-point is missing

If the user requests to --set-upstream-to a branch that does
not exist, then either:

  1. It was a typo.

  2. They thought the branch should exist.

In case (1), there is not much we can do beyond showing the
name we tried to use. For case (2), though, we can help to
guide them through common workflows.

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