bug#65308: (find-file) botches buffer-file-name in -batch mode

[Konstantin Kharlamov]

This was found while trying to use Emacs in `-batch` mode to regexp-
replace many files in a project. Turns out calling `(save-buffer)` will
make it say that directory does not exist, and the reason for that is
that `buffer-file-name` gets mangled filename.

# Steps to reproduce

        $ mkdir dir && touch dir/1 dir/2
        $ emacs -batch --eval '(progn (dolist (file command-line-args-left) 
(find-file file) (print buffer-file-name))))' dir/1 dir/2                       
             

        "/tmp/dir/1"

        "/tmp/dir/dir/2"

## Expected

Second print would say `/tmp/dir/2`

## Actual

Second print refers to non-existing `/tmp/dir/dir/2`.

# Versions tested

Reproducible with stable 29.1 and somewhat old master as of commit
04710bd01b2