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>

remote.c

@@ -1296,13 +1296,13 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 	base = branch->merge[0]->dst;
 	if (!resolve_ref(base, sha1, 1, NULL))
 		return 0;
-	theirs = lookup_commit(sha1);
+	theirs = lookup_commit_reference(sha1);
 	if (!theirs)
 		return 0;
 
 	if (!resolve_ref(branch->refname, sha1, 1, NULL))
 		return 0;
-	ours = lookup_commit(sha1);
+	ours = lookup_commit_reference(sha1);
 	if (!ours)
 		return 0;
 

t/t6040-tracking-info.sh

@@ -74,7 +74,7 @@ test_expect_success 'status when tracking lightweight tags' '
 	git checkout lighttrack
 '
 
-test_expect_failure 'status when tracking annotated tags' '
+test_expect_success 'status when tracking annotated tags' '
 	git checkout master &&
 	git tag -m heavy heavy &&
 	git branch --track heavytrack heavy >actual &&