`texinfo.tex` doesn't work with xetex

[Werner LEMBERG]

[texinfo.tex 2022-08-21.14]

Processing

```
\input texinfo

@documentencoding UTF-8

@include ae/lily-651af4bb.texidoc

@bye
```

(it's not necessary to actually have an include file) with

```
PDFTEX=xetex texi2pdf
```

aborts with

```
Bad character code (185937).
<to be read again> 
                   a
a->@ifpassthroughchars a@else a
                               @fi 
l.5 @include ae/lily-651a
                         f4bb.texidoc
```


    Werner