refs API: make refs_ref_exists() not set errno

Move refs_ref_exists from the legacy refs_resolve_ref_unsafe() to the
new refs_werrres_ref_unsafe(). I have read its callers and determined
that they don't care about errno being set, in particular:

    git grep -W -w -e refs_ref_exists -e ref_exists

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

refs.c

@@ -309,7 +309,9 @@ int read_ref(const char *refname, struct object_id *oid)
 
 int refs_ref_exists(struct ref_store *refs, const char *refname)
 {
-	return !!refs_resolve_ref_unsafe(refs, refname, RESOLVE_REF_READING, NULL, NULL);
+	int ignore_errno;
+	return !!refs_werrres_ref_unsafe(refs, refname, RESOLVE_REF_READING,
+					 NULL, NULL, &ignore_errno);
 }
 
 int ref_exists(const char *refname)