t1507: add tests to document @{upstream} behaviour

t1507: add tests to document @{upstream} behaviour

In preparation for future changes, add tests which show error messages
with @{upstream} in various conditions:

- test branch@{u} with . as remote
- check error message for branch@{u} on a branch with
  * no upstream,
  * on a branch with a configured upstream which doesn't have a
    remote-tracking branch
- check error message for branch@{u} when branch 'branch' does not
  exist
- check error message for @{u} without the branch name

Right now the messages are very similar, but various cases can and
will be distinguished.

Note: test_i18ncmp is not used, because currently error output is not
internationalized. test_cmp will be switched to test_i18ncmp in a later
patch, when error messages are internationalized.

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