Be more specific if upstream branch is not tracked

Be more specific if upstream branch is not tracked

If the branch configured as upstream didn't have a local tracking
branch, git said "Upstream branch not found". We can be more helpful,
and separate the cases when upstream is not configured, and when it is
configured, but the upstream branch is not tracked in a local branch.

The following configuration leads to the second scenario:

    [remote "origin"]
    	    url = ...
            fetch = refs/heads/master
    [branch "master"]
            remote = origin
            merge = refs/heads/master

'git pull' will work on master, but master@{upstream} is not defined.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>