Fix behavior with non-commit upstream references

Fix behavior with non-commit upstream references

stat_tracking_info() assumes that upstream references (as specified by
--track or set up automatically) are commits. By calling lookup_commit()
on them, create_objects() creates objects for them with type commit no
matter what their real type is; this disturbs lookup_tag() later on in the
call sequence, leading to git status, git branch -v  and git checkout
erroring out.

Fix this by using lookup_commit_reference() instead so that (annotated)
tags can be used as upstream references.

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