branch: improve error message for missing --set-upstream-to ref

branch: improve error message for missing --set-upstream-to ref

If we are trying to set the upstream config for a branch,
the create_branch function will check both that the name
resolves as a ref, and that it is either a local or
remote-tracking branch.

However, before we do so we run get_sha1 on it to find out
whether it resolves at all (since the create_branch function
is also used to create actual branches, it wants to know
where to start the new branch). This means that if you feed
a ref that does not exist to "branch --set-upstream-to",
rather than getting a helpful message about tracking, you
only get "not a valid object name".

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