checknlink: return False if .hgtmp file preexists (issue2517) If os_link fails on Windows, errno is always errno.EINVAL, so we can't really say if the testlink could not be created because (a) the FS doesn't support hardlinks or (b) there is a leaked .hgtmp file lying around from a previous crashed run. So let's err on the safe side, keep the code simple and assume we can't detect hardlinks in both cases.