Test tracking of non-commit upstreams

Test tracking of non-commit upstreams

git-checkout and git-branch allow setting up an arbitrary committish as
the upstream reference for --track. In particular, tags are allowed. But
they and git-status barf on non-commit upstreams as soon as they are
asked for trackings stats.

Expose this shortcoming by adding two tests: annotated tags are affected
but lightweight tags are OK.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>