for-each-ref: fix objectname:short bug

When objectname:short was introduced, it forgot to copy the result of
find_unique_abbrev. Because the result of find_unique_abbrev is a
pointer to static buffer, this resulted in the same value being
substituted in for each ref.
Signed-off-by: NJay Soffian <jaysoffian@gmail.com>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/for-each-ref.c

@@ -228,7 +228,8 @@ static void grab_common_values(struct atom_value *val, int deref, struct object
 			v->s = s;
 		}
 		else if (!strcmp(name, "objectname:short")) {
-			v->s = find_unique_abbrev(obj->sha1, DEFAULT_ABBREV);
+			v->s = xstrdup(find_unique_abbrev(obj->sha1,
+							  DEFAULT_ABBREV));
 		}
 	}
 }