bug#30215: Visiting files from zip archives should not modify directory time

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Cc: 30215@debbugs.gnu.org
> Date: Tue, 23 Jan 2018 23:29:34 +0200
> 
> It modifies the time of the directory where the zip archive is located,
> e.g. visiting a file inside ~/dir/file.zip modifies the time of the
> directory ~/dir/ because during unzipping in that directory, arc-mode
> creates a temporary backup with the file name from ‘buffer-file-truename’.
> Setting ‘buffer-file-truename’ later prevents from creating this
> unnecessary backup copy that modifies the directory time.

If that's the problem, then I think it would be cleaner to disable
creation of lock files and backup files by less subtle means -- by
let-binding create-lockfiles and make-backup-files.  Doing that by
leaving buffer-file-truename at nil leaves the code less clear IMO,
and relies on assumptions that might not hold at some future point.