git-add: adjust to the get_pathspec() changes.

git-add: adjust to the get_pathspec() changes.

We would need to notice and fail if command line had a nonsense pathspec.
Earlier get_pathspec() returned all the inputs including bad ones, but
the new one issues warnings and removes offending ones from its return
value, so the callers need to be adjusted to notice it.

Additional test scripts were initially from Robin Rosenberg, further fixed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>