Provide branch name in error message when using @{u}

Provide branch name in error message when using @{u}

When using @{u} or @{upstream} it is common to omit the branch name,
implying current branch. If the upstream is not configured, the error
message was "No upstream branch found for ''".

When resolving '@{u}', branch_get() is called, which almost always
returns a description of a branch. This allows us to use a branch name
in the error message, even if the user said something like '@{u}'.

The only case when branch_get() returns NULL is when HEAD points to so
something which is not a branch. Of course this also means that no
upstream is configured, but it is better to directly say that HEAD
does not point to a branch.

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