copyfiles: deal with existing file when hardlinking If the hardlinking fails, we fallback to `shutil.copy`, but do not consider future hardlinking doomed. This is a slight improvement from the current situation, we still avoid hardliking in a case we might be able to do it. However this does not have an impact of the rest of the operation. (This is an opportunity improvement while looking at something next to that.) Differential Revision: https://phab.mercurial-scm.org/D10841