do_one_ref(): save and restore value of current_ref

If do_one_ref() is called recursively, then the inner call should not
permanently overwrite the value stored in current_ref by the outer
call.  Aside from the tiny optimization loss, peel_ref() expects the
value of current_ref not to change across a call to peel_entry().  But
in the presence of replace references that assumption could be
violated by a recursive call to do_one_ref:

do_for_each_entry()
  do_one_ref()
    builtin/describe.c:get_name()
      peel_ref()
        peel_entry()
          peel_object ()
            deref_tag_noverify()
              parse_object()
                lookup_replace_object()
                  do_lookup_replace_object()
                    prepare_replace_object()
                      do_for_each_ref()
                        do_for_each_entry()
                          do_for_each_entry_in_dir()
                            do_one_ref()

The inner call to do_one_ref() was unconditionally setting current_ref
to NULL when it was done, causing peel_ref() to perform an invalid
memory access.

So change do_one_ref() to save the old value of current_ref before
overwriting it, and restore the old value afterward rather than
setting it to NULL.

Reported-by: Mantas Mikulėnas <grawity@gmail.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -630,18 +630,22 @@ struct ref_entry_cb {
 static int do_one_ref(struct ref_entry *entry, void *cb_data)
 {
 	struct ref_entry_cb *data = cb_data;
+	struct ref_entry *old_current_ref;
 	int retval;
+
 	if (prefixcmp(entry->name, data->base))
 		return 0;
 
 	if (!(data->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
 	      !ref_resolves_to_object(entry))
 		return 0;
 
+	/* Store the old value, in case this is a recursive call: */
+	old_current_ref = current_ref;
 	current_ref = entry;
 	retval = data->fn(entry->name + data->trim, entry->u.value.sha1,
 			  entry->flag, data->cb_data);
-	current_ref = NULL;
+	current_ref = old_current_ref;
 	return retval;
 }