fetch: fix regression with transport helpers

Commit e198b3a740 changed the behavior of fetch with regards to tags.
Before, null oids where not ignored, now they are, regardless of whether
the refs have been explicitly cleared or not.

  e198b3a740 (fetch: replace string-list used as a look-up table with a hashmap)

When using a transport helper the oids can certainly be null. So now
tags are ignored and fetching them is impossible.

This patch fixes that by having a specific flag that is set only when we
explicitly want to ignore the refs, restoring the original behavior.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -237,6 +237,7 @@ static int will_fetch(struct ref **head, const unsigned char *sha1)
 struct refname_hash_entry {
 	struct hashmap_entry ent; /* must be the first member */
 	struct object_id oid;
+	int ignore;
 	char refname[FLEX_ARRAY];
 };
 
@@ -287,7 +288,7 @@ static int refname_hash_exists(struct hashmap *map, const char *refname)
 
 static void clear_item(struct refname_hash_entry *item)
 {
-	oidclr(&item->oid);
+	item->ignore = 1;
 }
 
 static void find_non_local_tags(const struct ref *refs,
@@ -373,7 +374,7 @@ static void find_non_local_tags(const struct ref *refs,
 			BUG("unseen remote ref?");
 
 		/* Unless we have already decided to ignore this item... */
-		if (is_null_oid(&item->oid))
+		if (item->ignore)
 			continue;
 
 		rm = alloc_ref(item->refname);

t/t5801-remote-helpers.sh

@@ -301,7 +301,7 @@ test_expect_success 'fetch url' '
 	compare_refs server HEAD local FETCH_HEAD
 '
 
-test_expect_failure 'fetch tag' '
+test_expect_success 'fetch tag' '
 	(cd server &&
 	 git tag v1.0
 	) &&