refs API: make resolve_ref_unsafe() not set errno

Change the resolve_ref_unsafe() wrapper function to use the underlying
refs_werrres_ref_unsafe() directly.

From a reading of the callers I determined that the only one who cared
about errno was a sequencer.c caller added in e47c6cafcb (commit:
move print_commit_summary() to libgit, 2017-11-24), I'm migrating it
to using refs_werrres_ref_unsafe() directly.

This adds another "set errno" instance, but in this case it's OK and
idiomatic. We are setting it just before calling die_errno(). We could
have some hypothetical die_errno_var(&saved_errno, ...) here, but I
don't think it's worth it. The problem with errno is subtle action at
distance, not this sort of thing. We already use this pattern in a
couple of places in wrapper.c

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -1785,8 +1785,10 @@ int refs_init_db(struct strbuf *err)
 const char *resolve_ref_unsafe(const char *refname, int resolve_flags,
 			       struct object_id *oid, int *flags)
 {
-	return refs_resolve_ref_unsafe(get_main_ref_store(the_repository), refname,
-				       resolve_flags, oid, flags);
+	int ignore_errno;
+
+	return refs_werrres_ref_unsafe(get_main_ref_store(the_repository), refname,
+				       resolve_flags, oid, flags, &ignore_errno);
 }
 
 int resolve_gitlink_ref(const char *submodule, const char *refname,